The SPEC case is nothing "special" . The "run" was cast at Ford's specialty foundry most often used for short run , pre production prototype parts. The SPEC tag has been used on LOTS of parts. Water pumps , cylinder heads , intakes ( cast iron only) are some of the parts I have owned with it. I have yet to discover the reason that foundry was used but C4AW-B case production resumed at the Detroit Iron Foundry (DIF) on the castings in late '67 after the end of SAI production."My" guess is about 4,000 cases were cast there in '67 only. They are ONLY "correct" for the production vehicles Royce listed .
Randy