When Honda made the barrels they measured them and stamped the different sizes a or c. Some poor sod had to measure pistons which were also stamped to denote the actual size because they could make them all the correct size

I think that's what happened

Anyway the chance a barrel stamp still corresponds to the size is highly unlikely as it's probably worn or been replated.

The best solution is buying pistons then getting the barrel replated to match those new pistons rather than a replate then trying to get pistons to match

Don't forget this is 25 year old production techniques and Hondas crafty way of getting around the inaccuracies