In what year did the 45 RPM record come out?

Gnomon

It was in the period 1948 to 1950, but I don't know the exact date. Encyclopaedia Britannica says: In 1948 Columbia Records demonstrated 12-inch unbreakable vinyl discs that could play about 25 minutes of music per side. The standard shellac disc had revolved at 78 rpm, and a 12-inch disc had to be changed, automatically or manually, every five minutes, thus breaking up the continuity of longer {works;} the 12-inch LP, revolving at 33 1/3 rpm, could hold the average symphony, sonata, or quartet on a single side. And the vinyl discs had quieter surfaces than the shellac. Victor soon countered with its own microgroove records: seven-inch vinyl discs at 45 rpm. Mar 21 03, 3:05 PM