"Color socket"--what was it used for?

[Jeffhs]

Years ago, I had a book with service information on several makes of early 1950s black-and-white televisions. The rear chassis views of some of these sets showed what was referred to as a color socket; the service info called out the correct voltages to be found at certain pins on that socket. What was this used for? Did some TV manufacturers of the '50s make an optional, outboard color adaptor for their black-and-white sets that plugged into the color socket? If so, I would expect any color adaptor of that period would have almost had to be designed for the CBS color system using a color wheel, as the RCA all-electronic NTSC color system didn't appear until much later.

Thanks for any and all replies.