Super10 I worked at the Yamaha dealer in Memphis then Butler Yamaha, Triumph on summer we were the only Yamaha dealer in Memphis. I probably put this bike together out of the crate does it still have the dealer sticker. Do you know who originally bought it? Man I wish you could post pictures 
 Ps I still have a few of the stickers Butler Yamaha 

Yes, as I stated, I did know the guy who bought it; however, I had little personal contact with him. He was the sort of rich kid who sort of flitted from one activity to another. That is why I doubt that he bothered to build an MX from an Enduro. His family lived in Holly Springs, MS, 50 miles SE of Memphis. He went to MSU with me, another 120 miles SE. I graduated in 1971, so I do not know if he went home to buy the bike or if he purchased it locally. The back story is that I had a first year YG-1 from a local Yamaha dealer that I believe was gone before 1971, but I am not sure. He also could have bought the machine from some other MS dealer. I am purely guessing that it came from Butler, but knowing the story I know, Butler is likely. I can find no stickers on the machine. If you want to correspond directly, I can supply my email and send you all the photos you can stand!

So many of those bikes were "converted" to MX.... The VIN starting with "4" points strongly towards you having a non-USA Enduro.  Many were brought from out of country to the US by returning members of the Armed Forces. 

If you look at Yamaha's numbering convention, they have big jumps in the numbers between Enduro and MX.  Also note that in the US, all the AT Enduros were 12v generator/starter models...  RT325's Enduro has a magneto from the factory just like the US ATMX.  I've always suspected the AT VIN starting with a "4" was Yamaha's way of designating a "kick start only" magneto AT that was never sold in the US officially. 

High fender conversions were super common where I lived as well...  so common that I'm surprised any low fenders survived.  The mounting bracket and fender type can provide clues. 

Cleated pegs are almost certainly aftermarket or off a later Yamaha. 

I posted the wheel width for a reason.  Have you checked your rim widths?  Should have some stampings in them, often near the valve stem. 

Tires can be swapped so easily!  And now that you mention it, I'm pretty sure 18" front knobbies were not common if available at all.  A "serious" racer would have installed a 21" wheel kit that was available from Yamaha.  In fact, an early owner might have done just that and then swapped back to 18" wheel.  (I sold the 21" inch wheel off an early "converted" Enduro I had to a good friend and replaced it with a the original 18")

If you're really serious about figuring out what you have, figure out a way to post pictures.  There are some real experts on here on details and identification.  (I'm not one of them)

The wheels are Takasago brand 1.60 and 1.85. The front fender bracket is attached to the lower triple clamp with two bolts in front on each side and two from underneath (6 total). When I bought the bike the front high mounted fender had a crack in it, so I replaced it with a Preston Petty plastic one to keep from further damaging the alloy fender. I bolted up the plastic fender fitted to the same bracket.

Does anyone else have an AT-1 or AT-1 MX of any year with a 410000 serial number?

From what you've described about the fender and the rims, that evidence points towards AT1M. 

I wonder if RT325 can check his rim widths?  It could be normal that other countries had the wider wheels with the magneto. 

I did a bunch of research through my documents and found what appears to be a Yamaha document referencing an obscure "AT1J" model with a VIN number range of AT1-400101 to AT1-430000.  Markets listed were "Oceania" and "Japan". 

I could not find any reference to a US model with that VIN number range.

From what I can tell, the "AT1J" had the close ratio transmission as well, the 265 model code prefix for the "close ratio" gears links to the AT1J model.

The shop manual you have on the website states that the IBM number of the engine would be 263. You mention 265. Where do I find that number on the machine? You are probably correct about the J for Japan. Did Yamaha stop stamping Made in Japan in the left side engine case in 1971? It appears that they did.

i.imgur.com/7cxsUxk.jpg
imgur.com/7cxsUxk

I'm just old guy with this new-fangled Imgur thingie. I just joined and copied this photo. If you guys would let me know which link to use, I have lots more photos.

Model code for a US AT1M is 263...  you know the bikes that have VIN numbers that match the USA VIN tables. 

I found a reference to the AT1J having the "4XXXXX" VIN number range I posted.  Non-US model as far as I know.  Oceania and Japan were listed as countries.  (Oceania is Australia and surrounding areas)

Searching the Internet for "AT1J", I came across ebay sales for transmission parts for AT1J.  Part number (model code) prefix was 265, not 263 (AT1M) or 248 (AT1). 

So I looked up the close ratio gears for the AT1M.  The "close ratio" MX gears (that are different from the Enduro) have the 265 model code prefix. 

Finally I looked up the 265 model code prefix and it shows that code belongs to the AT1J.  That list shows the country as "Japan". 

So my conclusion is there is a very good chance the AT1J has what we in the US would call the close ratio transmission since the special close ratio gears use the 265 model code prefix.  You could also say the US AT1M came with the AT1J transmission. 

At this point my guess is you have an AT1J.  Or certainly a non-US model.

Without more info, it would be a stretch to say the AT1M VIN numbers went into the 4XXXXX range.