I’m working on my At2 the seat has been welded is ok but I would like to get another one from what I understand is the At1 is different than the At2,3 what is the difference my guess is the exhaust is mounted under the seat if that would make a difference does anyone know? Thanks 

Thanks Enduronut !! Have a good one 

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Well . . . unfortunately, after checking my bikes, the AT1 and AT2/3 seats are not a direct swap over.
The only thing I see that is an issue though, is that the rear hinge was moved further back on the later model.

However, I'm thinking a person could carefully cut the rear hinge  from the frame and re-weld it more forward (as per the AT1 location) and the earlier seat would than work . . . or . . . cut a hinge off a donor bike, and weld it on, while leaving the original rear "inner" hinge in place.

That way, you could use either seat until the correct seat is acquired .... then undo your mod by cut and grinding your temporary hinge off the frame,  Presto ... back to original again without a trace (if done carefully)  LOL

Thanks Enduronut for taking the time I guess they changed the hinge mount to accommodate the exhaust tab on the later AT’s again Thanks!!