They’re known by many names: 45 rpm record inserts, single record adapters, 45 rpm spindle adapters, spider inserts… all terms for the thingie that goes in the middle of an oldie.
All of these little doodads exist to solve a problem that the RCA Victor company created. They invented the 7-inch 45 rpm record, and added a 1-1/2″ hole in the middle to facilitate their automatic record changing device… and, well, 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: 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.