The service sheet for the RMK 721 is dated Jan, 1965, so it probably came out at that time or the previous year (just a guess.)
I suspect you did fine regarding the purchase price. Is it and the clock working okay? One dealer I talked to a couple of years ago said he couldn’t sell anything that didn’t do something more than just be a radio, as the space people had for objects in their living area was so limited. He said that he had more success with clock radios than just plain radios. People still liked to have them for alarms, etc.