I think what I can glean from that ad is that it receives CB channels and converts them to a bunch of individual AM radio signals so you can use the radio dial on a regular car radio to go through them. The spot switch I presume makes all of the AM signals play CB channel 19.
It looks to me like it’s a police scanner that pumps it’s output to a cb radio channel. I guess it’s for a driver who doesn’t want a CB radio and a police scanner cluttering his dashboard. I doubt it was a big seller, cause back in those days, drivers LOVED the flashing lights/LEDs of a police scanner.
The ad said it picked up 23 CB channels, though, which are not police scanner frequencies, so the ad sounds a bit misleading. The main value of the gizmo seems to be its ability to monitor channel 19, which is the frequency truckers mostly use. You could get information about where the speed traps are, road and traffic conditions, or if there are any accidents on the road ahead of you. If there were any police on the CB frequencies, they were using them informally and not in any official capacity, as they used VHF FM and not the AM frequencies of CB. As the name implies, CB is a citizens band.
It’s a CB reciever that one hooks up between the car radio antenna and the car radio. It allows one to listen to CB radio traffic in the area. The giveaway is the Motorola-type jack for the antenna on the back. CBs don’t use that kind.
A similar device appeared in the late 70s, when FM radio stations became very commonplace, but a lot of existing cars had only AM radios. The device picked up the FM band, retransmitted it to your car radio on two AM frequencies (in case one of them was occupied by a local station), and you could leave your car radio tuned to the same AM, while dialing the FM on the box. They actually worked quite well, and were amazingly cheap. A new FM station in rural Kansas bought case lots of them, and gave away 2,000 of them as station promotional prizes