Body broadcast sheets are full of two-letter codes. Box 103 is fender piercing: YA has been seen on both COPOs and Z/28s. Since neither received engine ID it may only indicate the "Camaro" script on the side. I have a copy of a Z/28-RS sheet and the code is YB.
More interesting is box 104 "emb". For Z/28 the code is G. The box is blank on COPOs.
Wonder if the "R" beside the "G" in the fender extension is for RS cars?
I posted a picture of the "XO" code inside my fenders and John Z mentioned that these codes were used for piercing of the fenders. I guess for RS cars since my car is a RS Z. Have you seen "XO" before? There was another 69 RS car in the paint shop the same time my car was there and it too had the "XO" markings inside the fenders. However this other car was not a Z/28.