After stirring up this detailed discussion, I am slowly concluding that maybe I am the third owner of this machine and not the second. If the person who purchased this bike whom I thought was first, he could have been no older than 17 at the time. I have literally researched his obituary to find his exact birth date and he was a few years younger than I had always thought. So maybe an older person did import it from Japan, Australia or New Zealand, and the high school (not college) kid bought it used. It is just too bad that that owner and the guy that connected both of us are now deceased.

Remember when I said convenience always sells? Everything I do know about this bike, as well as the trail bike culture of that exact time and place indicates that a previous owner rode it until the original rear Trials Universal wore out and replaced it with a knobby and left the front tire as it was. He raced it until it needed a bore job and was no longer running, then stored it with its original front Trials Universal, which is still on it. The wheel sizes, tire sizes, and many other details show this to be a factory MX bike. Where it was shipped to is another question. Why would anyone bother to remove the manufacturer plate on the steering head, buy a new set of wheels and tires, and change countless small details indigenous to the Enduro? These changes would not make it any faster. No, I think this is a late-production MX shoved out the plant door with a number of tiny, hidden, strange details like the VM26SH carb and the serial number series.

If you want to dig deeper into this meticulous madness, I suggest you put a call out to round up and identify all the 400000 series machines you can find. I bet there a lot more of them out there than you think....

P.s.: I have always been even more obsessed with the '60's Honda Scramblers than the Yamaha Enduros. I spent years untangling the details of those models. As a manufactured entity, the Scramblers are to Honda what the Enduros are to Yamaha. Each series was the first of its kind for the company from the standpoint of designing from scratch models specifically for the U.S. market. Believe me when I say this 400000 serial number issue is nothing compared to all the strange, detailed changes made to various Honda Scramblers from 1962 to 1975!

Just a randon thought looking at pic 6, from a safety point of view--it looks like the needle jet extension into the venturi sits very very high--like the slide would sit on it before bottomed out. Is it just a camera trick. Have you used it & is it just me? again lol.

 1968 Yamaha snowmobiles were only offered as duel carb 350cc twins , there problems with that arrangement so for the 1969 model year single carb. 
 Possibly a lot of overstock resulting ? 
There is a 1968 duel carb at the New Hampshire snowmobile museum wish I’d taken a picture of it
  This is an interesting topic, 

RT325, this carb has been untouched on the machine since I bought it in 1975. It just so happened that when photos were requested, I had taken it off the machine to replace a clogged pilot jet, which I just installed yesterday.

Remember I worked at Butler’s we sold AT1Mx do you remember Desoto Cycle Ranch in Southaven Ms. I remember going to the motocross races a lot there was a kid that rode one he was a fast excellent rider but he was deaf I asked his father how does he know when to shift he said from vibration. Enduronut do you think that Yamaha could have supplied racing parts,engines to privateers?
PS I raced there a few times 

Ht1kid, I appreciate your question. The privateer concept was actually one of the things that first crossed my mind before I found this board. Although the built for a foreign country concept has sort of taken over the discussion, I still think they could have been sent to dealers to promote the brand. I believe Bridgestone and Kawasaki have done that very thing with the BS 100 & 175 and the Kaw 100 and 250 (all were early dirt racers). My first thought several months ago, when I first noticed the lack of a manufacturer plate on the steering head was that this machine was a promotional gift to a dealer that he could not legally sell or title without the official plate.

From this stand point who knows I think it’s fun to try to figure things out. Our shop raced every weekend motocross, flat track etc Super Fred was our resident racer Yamaha sent us a TZ350 to race at Daytona. Yes it’s sounding like a overseas bike. 

The concept of "model years" was a marketing strategy that may have started in the auto industry?  Many motorcycle manufacturers made models that were produced and sold for many years...  possibly because of lower sales volumes?  (Sales volumes that of course "blew up" in the 1960's)

Back then for example, California listed the "year model" on car titles...  but for motorcycles they used "year first sold" on the title.  That eventually changed and by the late 1960's, "year models" became more commonplace in the motorcycle industry.  (California didn't change their motorcycle title protocol for many years though...  there are a lot of bikes out there that were originally titled using "year first sold" instead of "model year"...  eventually California simply converted "year first sold" to "model year" in their records...  so you can find a bike with a newer title that says it's a 1970 or 1972 (or even possibly 73 or 74) "model year" when it's actually a 1971 model year but was first sold a different year). 

I also don't know that "convenience" was the driving force behind the motocross models in the USA?  When I was a kid in California, there were "scrambles" and "hare and hound" races...  off road...  but fairly smooth with a small jump here and there.  Many of the 1960's and early 70's machines could be ridden to the race...  street gear removed if you wanted to (or possibly required)...  you could win the race...  then ride home on the streets.

Bigger, better, faster soon resulted in more challenging "motocross" tracks and while you might be able to race, you weren't going to win on a "street scrambler" anymore. 

A big factor in my experience was COST.   Special purpose (mostly European) motocross motorcycles cost a lot.  Like twice as much or more.  And they were comparatively fast compared to a "street scrambler".   Because of the cost, most bikes that raced near me continued to be owner-built stripped down originally street legal machines.  Many racers wanted a Husqvarna or a Maico or CZ or Bultaco, not many could afford them. 

There was at least two young adults in my neighborhood when I was a kid that bought new bikes and stripped all the lights and street gear and made them into "race bikes".  All the street gear went straight into the trash.  The days of commuting to the off road race on the bike you raced were coming to an end.

I remember what a big deal it was when Yamaha started selling pre-stripped MX versions of the Enduros.  It had nothing to do with "convenience".  It had to do with COST.  You weren't paying for a bunch of parts that would end up in the trash and Yamaha included many of the "go fast" parts that you needed to add to your stripped Enduro if you wanted to win!  You saved a lot of money and it made racing far more affordable for the average Joe.

Most racers would continue to modify their bikes as funds permitted....  spending a ton of time and money doing so. 

The "convenient" option was to buy, say a Husqvarna.  You would literally get a factory-level race bike that needed nothing.  I knew people who had spent more on buying and modifying their Yamaha than they would have spent on a new Husqvarna!  So there enters another factor...  the drive to "create and improve"...  There is a huge amount of satisfaction in improving and modifying machinery for many people.  Unfortunately these days that "drive" to "do it yourself" seems to be getting replaced by "convenience". 

Anyway, the mystery of this bike may never be solved...  certainly some oddities.  And yes, I also was wondering if Yamaha may have taken some "overstock" models from another country, stripped them, and sold them as "MX" models?  Another oddity is one of the factory Yamaha parts lists in the Tech Library  shows the AT1CM came with an Enduro cylinder...  but the MX pipe, carb, etc.  Typo?  Or did Yamaha do some sneaky changes to get rid of the excess piston-port inventory before the "new for 72" reed valve models rendered them obsolete?

Next step I would take?  Send a brief letter (or email) to Yamaha USA with pictures of the VIN asking them if they have any record of any 4XXXXX series bikes that were officially sold in the US as Enduro or MX models?  They might come up with some new information. 



 

Looking at your post #62 reminded me [made me laugh] i started in a Honda shop In "62, maybe "63 but looking in the shop in "62 & the 125 Benly twin was on display.
Me being "Mr Know It All" said to my mates [who were ogling it] that it must mean it has 125cc each cylinder because how the hell could you possibly divide a 125 into Two Cylinders. [Me having 125 hand change Excelsior at the time lol].