On all the C4's I have had, the build sheet is glued to the underside of the cargo compartment door behind the passenger sheet. These are codes only, and the listing of the codes is found in the 2 vol. service manual.
You are all correct, the build sheet is glued to the top of the fuel tank, been there since the dinosaurs, as far as I know. The other sheet, with the build numbers can be located in different locations depending on the year. In my 91, and 93 they were located on the inside of center console door. My son's 96 has the sticker located in the rear storage compartment on the side of the bin opening.
My 2002 Z06 has them located in the inside panal of the glove box.
I can't remember where they were in the 60s, 70s, and early 80s, I'm lucky I can remember where I parked my car in my own garage.
At any rate these are important documentation of the original equipment, and info for obtaining correct replacement parts.
Your better off to leave them there and order a set from the Museum , I think it cost 35 bucks. OF course last year when I had my LT4 I contacted Sonny Kilgo (Since retired) and got three copies 8x10 size and one 20x15 for free. :crazy