It's colloquially called a "flattop", and was on nearly every four door hardtop from GM in that era. Even Corvair. Decidedly NOT rare. Cosmo
cousin used to have one. had alot of interior space. kinda feel like a roof like that leaves room to rot because of lack or pitch and areas where water could collect.
You could get this roof or a 4 door post roof, so the ones you've seen in the past were probably 4 door sedans. I like this roofline myself.
these were commonly refered to as "vista roof",exclusive to the 4dr. hdt. models. that i do believe was from the GM sales brochure's the salemen carried. love the style and 59'-60' both shared it.
Saw one in a demo derby years ago, rear end t-boned in the passenger side, those doors just broke away......