I found the original part number -1AMA1POS0.
I've put the damn chain back on 4 times now. The second time it came off, I noticed the rh sprocket was on backwards, fixed that. Today for the 5th time it jumped off again. I made a new arm for the idler that's longer than the original, figuring the original didn't take enough slack out.
No dice, second board it jumped off again. (Last picture). One damn dumb engineer who put the idler on the drive side of the chain instead of on the slack side. Not sure how, but I've got to try to get the idler on the bottom of the chain, not the top. (Chain rotates CCW).
Did I mention it's a challenge to get the chain back on?
Alexis de Tocqueville was a very smart man.