I did check behind the seat but I don't see any crayon marks, but if they weren't putting the x code on the cowl tag yet would they write it behind the seat?
Yes, Jerry M. found it on my brothers early pre X coded Z when he had it inspected a couple years ago. Put a very bright light on and maybe a use little soapy wet rag to lightly clean the area. If that area has never been blasted, repainted or replaced it could be there. I watched Jerry do it. You have to look very hard on his car to see it but it is there.
Edit: Bryon could be right, I think my brothers car is a Oct. build if I remember correctly.