Hi - have 2021 industrial 12 valve and 1990 12 valve. Can I switch the pistons on oil pressure relief valve between the two or do I have to switch the whole housing as well?
The 2021 has four holes and the 1990 has two holes on the piston. The spring on the 2021 is a tad longer as well.

 

Oops - didn't take into account what the OP said about the physical differences. Given that, I'd do some very careful measuring with a dial caliper that reads to the .001" if considering swapping pistons. If it was me, I'd probably swap the housings to be safe.