I don't know what the numbers represent on the domes. But I have a pair with a 7, 4, 6, and 8. Now the 6 could easily be a 9, as there are no other marks. These are some of the very first stuff Tom @ Pro Design ever put out, so I would call him to find out what the numbers represent. They don't look like they all represent the last number in ccs. They are all Pro Design manufacture and in good shape, with the exception of a blown cylinder on a few where the particles from the piston blasted it a little (can be polished out). I do not have any flywheels or round axle carriers left. Sorry. E-mail me and I can send pics of the domes.