Hello!
It's more than likely a 1976 GM car with a Delco radio with the CB built in was installed in the car at a later point as an update by the owner. We all make improvements - - I recently fitted alloy wheels from a 1995 Fleetwood Brougham on my 1981 Fleetwood Brougham to give the car a different look.
Here's the description from the 1977 sales brochure: "AM/FM Pushbutton Stereo Radio with Citizens Band -- This exciting new radio is available for the first time for Cadillac..." It's interesting to note that the original 1977 version of this CB only had 23 channels! The 1978 had 40 channels.
Looking at the sales literature, the 8-track/CB combo radio was introduced in 1978. The electronically tuned receiver (ETR) radio was an option starting in 1978 and standard in 1979 when cassette, 8-track, CB, and 8-track/CB versions were all offered. 1980 was the first year of ETR cassette with CB.
Breaker 1-9!
DAve