All of these little doodads exist to solve a problem that the RCA Victor company actually created back in the late 1940s.
That’s when the company released their invention: the 7-inch 45 rpm record. Instead of using the standard spindle hole size found on larger records (like the now-common 12-inch vinyl discs), they put in a 1-1/2″ wide hole in the middle to work with their automatic record-changing device.
The innovation came with the fringe benefit of cornering the market, as the little discs could only be played on their new line of phonographs. (See how RCA Victor launched the new-style phonographs here.)
So just because singles worked with jukeboxes and on many small record players, how could people with standard turntables make them play?
The answer was a little plastic (or metal) inserts that snapped into the large hole on a 7″ 45-rpm record single. With this simple addition, these seven-inch singles were compatible with the spindle on a 33-1/3 rpm (LP) record player.
Invented by a guy named Thomas Hutchison at the request of the RCA Corporation in the sixties, this inexpensive piece of plastic (or metal) helped bring music to the ears of millions.
Just snap it in the middle of your 45
I never realized there were so many designs of these! When I was growing up, record stores would often offer these for free at the checkout counter. Most record players had a built-in adapter so you didn’t always need them, but they were particularly handy when using a player with an automatic changer that would drop records from a stack one by one as the previous record completed playing.