I looked at the other 2 - the 03D Pace Car is like the 01C X77 - stamped GM and part number. The 12A '68 built L78 is different - lot of surface rust, appears either to be an AAc or triangle marking, I'll have to give it a bath in Evaporust or the old electrolysis routine to be sure, but it is different. All were built in Norwood, might be a vendor change somewhere in the first part of the year at Norwood. Need more examples to find a pattern -
Regards,
Steve