From the underside, probably looking at a raw forging/raw casting number that is then machined into several different finished parts, given different part numbers, and then
put into a package of 8 with wrist pins and so on and assigned different part numbers again.

I'm sorry that's not helpful at all but it may just be an internal ID number for the piston mfg, and no good to you and me.

Rich H.

Esse Quam Videri