Technical Eddie Meyer head info

I recently looked at a pair of 21 stud Eddie Meyer heads that have numbers stamped on the top edge .Both heads have the same numbers c121 and 500.Can anyone tell me what they are in reference to? Thanks

 

I'm still very much a newb, but perhaps the 500 is referencing valve clearance? I know my Offenhauser heads are stamped "400" on the top edge, referring to their valve clearance. But 0.5" of valve clearance seems larger than "normal" and would likely result in a fairly low compression ratio.

 

Check this out: https://www.jalopyjournal.com/forum/threads/eddie-meyer-heads.952069/

 

I think those .400-.500 numbers refer to the nominal amount of lift allowed and have nothing to do with clearances.

 

Yeah, instead of "valve clearance" I suppose I should say "valve relief"