If you’re old enough you might remember seeing one of these in a drug store, grocery store or even Radio Shack. You could pull all the tubes from your radio or TV, put them in a paper bag, and take them down to your local store to test. And hopefully you wrote down which one came out of which socket. Once you found the bad tube or tubes, the store proprietor would unlock the bottom and find new replacement tubes. And the price list is taped right inside the door.
67 here. I played around for hours with old tubes at the closest “variety” store by me. Couldn’t afford new tubes even when I found a bad one - I was weird, and it was something to do.
Occasionally I’d see an actual TV repairman use it, otherwise it just sat there.