Interesting topic. I have a matching frame and engine AT1 436421 and am in AUS. It currently has the non electric start crank.
Just a possible thought on this. The RT1 and DT1 had some close ratio gear models with the numbers been in a very different range and I wonder if these AT1 numbers were a close ratio range?
Mart T mentioned a link between the AT1J and 265 prefix which is for the MX gear prefix.
Enduronut also touches on the possibility of a way to get rid of stock.

There is possibly a bit of overlap with the below:
There are 2 DT1s that look identical to each other in that the numbers possibly have been converted from a close ratio model - possibly sold as a MX model. Numbers have been added to change the serial number in the exact same way. Both bikes have many MX parts - possibly a way for factory/dealers a way to get rid of MX and close ratio stock. One is linked below

www.yamaha-enduros.com/index.php/forums/...on/8540-vin-research

Unfortunately I can't open the other forum post, but maybe Mart T can if he has the time? Please Mark if you can. It was under Bike Restorations, Re:Bike Identification as the title, with dates 19 Feb 2022.

Anyway - food for thought more than anything factual.

With all due respect, Mark, the thing that has me so befuddled is that I have the same documents you do and the AT1J referenced shows a USA destination for the 400000-430000 range. The range is left blank for models sent elsewhere on the second AT1J line. If you scan through this list, you will see that other models such as the R3 and AT2MJ (#345) display a similar pair of lines, instead of only one. I have continued to research this issue since my last post. I spoke at length with Dale Sweger, who was surprised that frame numbers matched engine numbers at all, but this has been easily established. I have also communicated with the restorer of #419614, who thinks both our machines are domestic factory MX models.

After examining countless components, part numbers and photos of other machines, I have no doubt that my bike is a factory MX, not some sort of converted Enduro. However, I do think the J stands for Japan and the machine was originally sourced as a domestic model. That is why it has the welded on housing for a steering lock, as also does #419614. What I think happened is the factory had produced and stamped a batch of MX models for Japan, but U.S. demand caused them to ship a final batch of 1971 MX models to the dealers. This may have happened because the AT2MX was not yet quite ready to ship? Unfortunately we do not have a date of manufacture because both machines are off-road only.

1960Super10,

Maybe you're responding to someone else or a post I've forgotten about?  The document I have clearly shows the AT1J only as a Japanese model.  I can't recall ever seeing anything else.  I would be interested in seeing what document you have if you can upload to imgur or some hosting site and link here.

I also believe I previously expressed the same possibility that Yamaha imported a few "J" models as "MX" models?

Some people believe the world is flat...  just because a more than one person shares a belief, doesn't make it "the truth".  That's the thing about the truth....  every person on this forum could believe your bike was made on the Planet Mars using slave labor from Venus.  It wouldn't make it true.  The truth of the bike's origins will still be the truth even if nobody believes it.  The truth doesn't care what we "think".  Only things that are not true require energy to promote them or they will vanish.

Enough "philosophy". 

I'm pretty sure I've already suggested this but did you ever send a letter and pictures of the VIN and bike to YAMAHA USA?  They may have records that confirm your theory!  That would put the matter to rest.  Even if they have nothing, you're no worse off than you are now.  (Yamaha used to be very responsive to inquiries like yours and always replied by mail to the people I know that took the time and a stamp to write to them...  I doubt that has changed)

 

The document in question is the Model Index found in the documents on this site. Yes, I did send a letter to Yamaha months ago and they never responded. For now I shall assume the machine is a domestic market MX and quit trying to figure out exactly how it got here.

I see the model index entry you mentioned, thanks...  I usually use the earlier model code list as there does seem to be errors and inconsistencies in the model index...  but that's true with most Yamaha docs. 

Earlier model code list shows it as a "Japan" model only.