I have had that question posed to me, and here is the info.
A TV test jig was a CRT in a case with adapters that allowed you plug in various makes of TV chassis to it so the outside tech didn't have to bring in the whole set, cabinet & all when servicing was needed.
Being able to leave the yoke undisturbed on the customers set was a blessing, as you did not have to reconverge the set when you brought the chassis back. That could eat a lot of time in the home.
They were made by many companies. I still have my RCA, Sylvania, Zenith for color and 3-4 for B&W TV's. The color were typically made in 19", 21" 25" and IIRC, there was one even larger made.
The B&W were typically 6, 7, 8, 9, and 12 inch CRT's Pretty vanilla. Just extension cables for the HV, CRT, and yoke.
They typically had a yoke impedance matcher, yoke extension cable, HV extension cable, CRT extension harness, and a set of plug in "dongles" for each chassis available. Some even had a speaker and extension wires for it. You had a certain number of "dongles" that came with the jig, depending on who made it.
You did need to buy the chassis "dongle" sets separately, and IIRC, they ran from $15.00-40.00 each.
The CRT assy. was often made to be hung from the ceiling on screw eyes, or set on a shelf.
They were a 2 person job to hang from the ceiling too. HEAVY!
Some of the color ones had all the connections, including the convergence panel easily available from the front of the set. Others required you to have to plug in connectors to the back.
The really expensive ones even had HV metering on the front too! That made it very easy to monitor it while troubleshooting.
You also received a hookup book with the jig, and updates were available to tell you which chassis used what set of "dongles'. They would work on other brands than the companies that made the jigs also.
You can still find them here and there, but the books always seem to be missing, and the adapters/ dongles are often readily found at ham fairs, with the seller almost never having an idea as to what they are.
RCA, Sylvania, and Zenith were the most popular in my area, and the "el cheapo" ones 3rd place.
Personally, I liked the jig that matched the chassis (RCA for RCA chassis, etc.) I was working on at the time, as the mfgr. always did the best job of matching to their own product.
Now, if you are restoring an earlier tube CRT set, and you find a test jig that the CRT number that matches your TV set, typically the CRT will be in "Like New" condition, as they usually had very few hours on them, and unless the vertical failed & burned the CRT, they are a cheap and easy alternative to finding an expensive, good used CRT on the cheap.
You did have to pay attention to the HV also, as some TV sets used up to 40KV anode voltage, and the majority of the jigs would handle 23-25KV OK.
Some of the test jigs would handle solid state as well as tube chassis.
EDIT: There was a test jig, well, OK sort of.... Made by Pic-Test. It was only for B&W sets using the Duodecal (B12?) base. That is to say, it would not fit the very early CRT's (DiHeptal?) and the later B&W glass base CRT's.
It used a 6AL7 dual section eye tube. IIRC, it was extremely simple and basically only told you that a signal, filament, & B+ were getting to the CRT, by the eye sections action. It did not test voltages for accuracy, ripple or anything else.
It is more a novelty than a test jig, but is interesting. I scored one many years back, as an addition to my "Odd Stuff" collection.
So if you find one, it is likely to look like a Duodecal CRT socket attached to an octal base, with a 6AL7 in it, you found one. If you are very lucky, you will find it in the original round cardboard "tube" with the instructions. In the can, it is a bit bigger than a small tomato paste can.