I’m used to those funky speed test strobe mats which hi-fi shops used to sell on the 70s and 80s, you’d slap them on the deck then check to see if the turntable was going at the right speed (unless your turntable was posh enough to have one printed on the side.). I have a few which I must photograph for the site. But I had not realised these went back quite so far, until I spotted this 1920s HMV sleeve, advertising just such a speed tester. I’m not sure how it worked, you placed it on the spindle during play and it seems to have some sort of little indicator on it which I assume was moved by centrifugal force and lined up when it was at the exact speed. Nor do I know what you could do about it if the speed was seen to be out. I did find a photo of one (see below) which sold at auction for over £130.
We have also covered some hi fi audio test album sleeves on the site.