I agree that there are few "100%" rules with vintage Heuers...but in the normal course, the Camaro case should have the reference number between one set of lugs and the serial between the other set. There are several reasons why it may not be there:
- Was polished off/ worn away
- Replacement case (original damaged and replaced by Heuer)
- "Parts" watch- remember that Heuer also offered the Camaro case to other brands (although even some Zodiac Camaros I've seen have the Heuer numbers between the lugs)
Maybe there are some other reasons why it's not there, but in terms of what's "normal", it's definitely normal to have the serial and ref numbers