The all red tail lamps ceased in mid 1969 I believe? My car built in January of '69 had all red until I changed them out.
I also noticed that mine are one piece, I also have the remnants of many others that look like they were in multiple pieces; the chrome bezel being separate.
One other thing: I've seen where new tail lamps don't fit old bodies all the time...I saw this myself on Bob Geco's 1968. I did not have any issues fitting new to old on my car, but Bob's "out of box" experience was not pretty--which is why they are in the box still, waiting for an inevitable paint job when he can do the necessary body work to fit the new ones.
Ulf has a great point. I have quite a few broken tail lamps inherited with the car. I guess my Uncle the PO had been rear ended many times.