Need help ordering parts

[Biddington]

I am in the prosses of ordering new pistons, rings, and gaskets, but im not 100% clear on if im ordering the correct parts. the bike is a 1973 tx650 engine numbers S650-213923. code 366. everything on mikes that "says" is for a 1973 uses code 256? any help / clarification would be great.

thanks,
stephen