I should rename it 'core revision number' as all these blocks were sand cast as there was no die cast mold that iron was poured in repeatedly. I would think every block cast would have its interior and exterior cores destroyed in the process so there is no wear on the disposable cores themselves. The molds that make the cores are what gets worn down and the exterior texture is probably due to the fact that the core molds get filled with abrasive sand/glue, cleaned out and dusted so often that the surface finish starts to decay. They would not have 1 mold for a blocks cores, they would have several (12?) so they could get these onto the line fast. Each core mold would have its own control number for tracking purposes. If core -8 were found to have a bad tolerance, it would be tracked in all the -8 blocks. I'm just looking at this from a student of manufacturing process perspective. As for the 340 resto block finish, those core molds were probably lower production (in the grand scheme of 340 production) and were subjected to less wear. I would think the 'first pour is less porous" may be more of a die cast term as those die castings do wear over time also.
