I have seen many guys with original cars from may, june july and august state they still had the original 4 clip design. Enough guys, that it seems more likely they were original to the car, than somebody went and located the 4 clip and swapped out the finger design. Just my 2 cents.
My original 09C Z28 which I purchased in 1976 had the 4-clip covers on it when purchased; they had the normal dings/scrapes in them, so I tried to purchase new ones (but the 4-clip design was not available in the 15x7 size at that time so I chose not to buy the 'wrong ones'). The 'finger style' trim ring WAS available OTC during those years, as that was what they were installing on the '73 to '77 MonteCarlos (all having 15x7 wheels), and El Caminos/Chevelles with 15x7 rallyes. I purchased a new '77 MC, and sometime in the late '70's, I actually swapped the trim rings between my MC and the Z28 (because the MC trim rings were like new and I wanted the 'best looking' ones on my Camaro..
. Fortunately, I traded them back before I sold the Monte Carlo.
It's interesting that during that same period, Corvette trim rings (15x8 size) ALL had the 4-clip style, both on new Corvettes, and OTC. I have a set of them that I bought around 1977 when I put Corvette 15x8 wheels on my Z28 (temporarily) until the front fender scrubbing got to be too much for me. IF the 15x7 rings with 4-clips had a tendency to 'fall off', then why did ALL the larger 15x8 Corvette wheels have that same 4-clip design from the late '60s thru the mid-late '70's??? I suspect they began to use the finger design on the higher production cars - MC and Chevelles etc - in the '70s because the 4-clip designs are more difficult for owners to remove...