60' sounds closer,I have a piston with simlar numbers they were etched not stamped, one set of numbers went one direction and the second set went the other way like an L'it was simply a part No. It was etched in black and mine is
RP2063
F30
TOP 008 LF
So mine I have already measured as 30" The RP2063 is seemingly the Pt No for an A/M Victa 125 Piston,The F60 is for 60" O/S... So Im pretty sure your bore is Cetainly OS 60" which rings are still readily available. A simple measuremnt will clarify 100%