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.)
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.