I do not want to be a pest, but several of your spec pages are showing 3.00 x 18 and 3.25 x 18 tires for the AT-1M models. These all had 3.25 x 18 and 3.50 x 18 Trials Universals. My machine still has its original front Yokohama 3.25 x 18.

No plate on mine just stamped into the steering head neck.

RT325, is your bike an Enduro or Motocross?

Again, I do not want to be a pest, but does anyone else's first generation Enduro or Motocross have cleated footpegs?

I forgot to ask RT325: does it say Made in Japan on your left side engine case, just below the clutch cable?

Yes i shoulda said, Mine is an Enduro with original rubber rib covered pegs, what's left of them.
Yep it has made in japan where you say.
I'll try & link some pics & a video for a laugh [everyone else has seen it haha].
I won't clog up the post with the video but link it to the page it should be on.
Bike is "Rough As".

4960Super10...  I'm just curious what makes you so certain you have an AT1M?

Hope links work.
photos.app.goo.gl/xcprrjDdGtzu69XN6
photos.app.goo.gl/751PQrc5rHixYkRm8
photos.app.goo.gl/UbWTkigoNNXXeZze6
photos.app.goo.gl/uKL1ujBFjwVgZSiaA
Way down the page in the link below for vid. Can hear clutch slipping but stopped slipping after a blast around the track.
Clutch cover never ever been off [obviously needs fixing properly though].
yamaha-enduros.com/index.php/forums/gene...linder-is-this#83998
yamaha-enduros.com/index.php/forums/perf...-vz-ct-dt-175#163169
Linked another vid around the paddock [save filling the page with my irrelevant stuff]. Down the page on link above just drag on to about 2.36 for a clear acceleration past camera & no clutch slip, bit of wheel spin [in my dreams lol]. Not worth sitting watching it all--bit like watching grass growing.
Be good to see yours running sometime.
Found a clear pic of the frame number.
photos.app.goo.gl/HL1QUAZdBRasnETH7
 

MarkT, I am not so certain, but all the evidence is very strong that it is a factory MX. The first owner was not the type to build a racer from an enduro; he would have just bought the racer new, likely from a Memphis Yamaha dealer. I knew the original owner (now deceased) and the mechanic who rebuilt it for me back in 1975 was quite knowledgeable. His career was working for Honda, traveling around the country training their mechanics on new techniques. He was an old friend since college and the two of them were roommates. All of this is circumstantial, of course.

The machine still has the original Yokohama Trials Universal front tire in the 3.25 width. The front fender has the high mount. There is no sign at all of any electrical gear: wires, switches, connectors, battery, etc. It has cleated footpegs and the original type stinger. It has two spark plug holes and what seems to be polished side cases. The original racing owner had added a kill button. It is very quick.

All of this leads to why I am so obsessed with the serial number. I see above that RT325 with the other 400000 serial number responded. Since his machine is an Enduro, then obviously the 4 does not denote an MX. Cut to the chase. If this machine had been born an Enduro, the front tire would have been a 3.00. I think the likelihood that the racer would have changed to the wider tire and kept the Trials Universal tread is zero. I used to live where he did and he was racing in mud, not desert.

Yours would definitely be piston port inlet then being "71?.
Recently there was an AT2-MX for sale not far from me but i [tried to] ignore it lol. Was a total rebuild job & pretty rough plus i didn't need it at the price [$300]. Had the left engine side cover missing & who knows what else. Looked like one of my bikes lol. Wish now i'd saved some pics for this discussion. This one had reed inlet of course.
Looked identical to this--ok this is an AT3-MX , But same horse.

tinyurl.com/2wdre5hd

Yes, mine is a piston port. Torque Induction (reed valve) began with the AT2.