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.
I was in Georgia (the country) a few years back and visited a flea market, where, to my delight, several guys were selling a lot of Russian made tubes. I was inclined to buy a few, but didn’t have an opportunity to research the models on the spot, plus the hassle of taking back glass tubes in my luggage was eventually not worth it.
Still, it’s endearing to know that they’re still around, apart from maker projects and overpriced retro hardware.