After going through many threads on this topic I realized that in regards to a specific year there has never been definite answer as to what year the Norwood plant changed the primer color formula or better said the orange tint vs brown tint. It would be nice to put all the information together for future use, or for lack of words-put to bed. My 1974 LTZ28 which is original has the brown underside along with interior portions. From what I gather the shells were completely dipped. This subject has come up again in the Project Progress section by secondgenaddict during restoration of his matching numbers 1973 RSLTZ28. He uses Pantone color chart to perfectly match the underside color of his 1973 which is the same as my 1974. From my extensive research this is what I believe to be true: 1970-1972 Camaro's used PPG DP74LF, Dupont Imron 62-711, OEM paint 42030 and Zero Rust -red oxide. 1973-1978 used something else. It has been suggested Dupli-color Acrylic enamal DA1656 leather brown is a very good match (comes in rattle cans only). I have not found enough information 1979-1981. If you '79-'81 owners are restoring your car to original form, please chime in. I can only assume at this time the color is the same as '73-'78. I'd really like to hear everyone's thought on this. The term "red oxide" primer can only really be used for '70-'72 Norwood undersides.