ID casting numbers on 401 pistons

[willysworker]

I just tore down a 401 and the pistons have casting #'s inside 3215756 and yo or oy. Can anybody ID these?
Also the rod bolts have 12 point nuts on them. Is that original?
Of 5 AMC's I've torn down this is the first with 12 point nuts on the rod bolts.

[1980_Cj7]

Both our 401's had the 12 point rod nuts. Can't help you with the piston numbers though.

[willysworker]

good to know. I was hoping this motor has a std. bore.