Those are 383 pistons which have 1/16,1/16,3,16 ring grooves.
They are 12.5-1 with a 78cc head.
Old school,heavy,powerforged pistons.
I run them with a full floating pin,non-bushed.
If you want to run a more modern designed,lighter choice,let me know. I'll buy 'em from ya.
John