Interesting how we miss things. There are at least 4 cars on eBay at present with this bumper guard. The bumper is the stock unit. The guard was an option and is bolted to the bumper. It was available as a factory option or dealer install. Just $19 in the 1st generation Chevy II years. I had one on my '63 SS convertible. It breaks up the flow of the front end a bit, but offers protection to the very flimsy aluminum grill.
The front was actually called a grill guard. Rears were bumper guards. Yes, most guards leave marks on the bumpers when removed. I liked the way it looked on the front of my '63 SS convertible. Picture below.
Yes, it was a nice car. Highly optioned with PG, PB, PS, a/c, comfort and convenience, front & rear bumper guards, and tissue dispenser. Interestingly, it had a manual top. A financial guru once told me, 'don't fall in love with your assets'. I always remembered that, so when the right price was offered, I sold the car.