Yes it is! The question nagging me for years is why the foundry changed it's practice of using that area for the casting date. My friend Kerry Wortman has narrowed down the date of time these were cast in ( '67 only) but finding a Detroit Iron Foundry worker that can say "why" it was done has not happened. The SPEC tag has been used on cylinder heads and other parts and so far the only tangible possibility (IMHO)was these parts were cast at the small run "specials" foundry , not the huge Detroit Iron Foundry that normally did them. Again "why" hasn't been answered. I know there is a specific reason that will get a "of course , that's why" response but it remains "to be discovered". My "educated guess" could also explain why a single rib version exists.
Randy