Ever? Yes. What was coded and when it was coded varies by plant so, yes, there are certain plants and applications that do show codes under the 1 to 8 numbers.
So R doesn't always code for radio? X for glass tinting? PNT color codes are not consistent across plants?
Well, for codes like body belt moulding (3-0) and wheel lip moulding (7-8) I don't have those codes on my 67 Monaco tag, yet I do have the 1" wide chrome body belt side trim and the wheel-lip trim. I thought those side-trim codes were listed on line 1 under the letters a-j. I don't think I've seen the number codes used (on the 3'rd line) on any '67 tags I've seen posted, but yes I see them plenty of times on the '68 tags.