Brian, Brian. LOL. Wonder Who gave you that info.
The
ribbed for your pleasure only applies with 455's.
The 400 are a totally different ballgame. There is no secret numbers or code to look for.
FWIW I have a coded RA IV block that I'm working on that doesn't have the support of a later 400 block.
Ya just have to have a supply of blocks and after awhile you will notice differences and what to look for.
This can go on for-ever. LOL.
Personally, I have found 4 different basic castings, as far as main area support, for all 400 Pontiacs.