Pontiac 400 engine head yellow chalk (?) marks

Dear Antique Car Enthusiasts,

I have a '68 400 block in a nice condition (e.g. factory paint, etc.) and have two cross- or T-shaped yellow marks on its left head. A former engine plant worker stated: "The people working would have used yellow to let the inspector or repairman know there was a problem." Is this info correct?

One of my friends and me are currently restorating a '68 Executive and a '69 GTO and we are trying to get as close as possible to the factory condition (and a '69 GP is also waiting for some precise work). Unfortunately even Paul Zazarine's Pontiac GTO Restoration Guide, 1964-1972 has only cursory informations relating paint and chalk marks (e.g. on the firewall, block, heads, etc.) for a good restoration...

Are there any better literature or an experienced pontiac expert who could help us?

Thank you very much in advance for any info!

Frank Lovas