256 is Metal Slug 3, and the ph1 and ph2 files belong to the set which has decrypted C roms and encrypted P roms. You could try renaming the P roms in your set to ph1 and ph2, but that might not (or then again, it might) help. Post the contents of your 256 set just in case. The board rules make it clear that you can't ask where to get rom files, so you'll have to find those yourself.