I recently cam across a 502 gen vi standard deck short block. With forged internals and srp pistons. I was in the process of replacing one of the pistons and discovered the piston number as 281919. Which would be .030 over making it a 4.500 bore 509ci i believe. But when looking it up on SRP they list it as direct replacement for OEM GM 502's.
Leaving me with the question whether GM made any 509's or was this once a 502 just bored .030?

the 502s came with a j&e piston.i would say yours has been bored .030 by one of it,s previous owners.

409's yes no 509's as stated above

here's something to think about...GM marine engines are not assembled by GM, they are subbed out to a independent source, and have seen weird parts used..like scat cranks in "factory 454" engine(own 2) so if the builder found the block not decent...does happen in even new blocks, maybe they bored and shipped out....not saying I have seen that, but I have found bored GM vehicle engines that have never been out since day 1...so who knows ????