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.
He owned a lot of them and sold a lot of tubes every month, generating revenue. Not sure how much, but with solid state becoming more popular, the amount kept going down, so he sold. Not sure who the German company sold to, but I assume they went to a country that was behind the curve on technologies and still had a large number of tube electronics in service.
There is still old equipment that needs repair, and radio shops restoring old equipment for audiophiles…so I can see these holding value