So is it safe to say that letters with C5ZB on the back are original Assembly Line pieces?
Would NOS or Service parts still carry that cast in number or not at all?
You just stumbled into the dirty little secret of the Ford numbering system. The C5ZB-16D0042-A thru G numbers cast into the backs of the (MUSTANG) letters are the real honest to goodness Ford part numbers. There is a Ford document (or documents) that tells how to make, in this case - cast, each letter and, even more important, how to mark them. That Ford document is often called the "engineering document" (phrases also used are "engineering drawing" and "engineering part"). To be "legitimate" a Ford trademark must/should be present as well. (This Ford document may, or may not, indicate the next higher assembly - the uses.) The service number, formatted C5ZZ (the last "Z" indicates it's a Ford Service
STOCK NUMBER) -16098-E thru L (no letter "I"), is nothing more than a sheet of paper (or computer entry in today's world) that says "USE C5ZB-16D042-xx" (for which ever letter is required) to make the part.
Now you know.
Jim