Back to Frink Street! Google only turns up one hit for Thayer Prop Shop on Frink, but from the name, presumably it specialized in theatrical props. It doesn't seem to me that would be a large market in the Columbia area, so perhaps they did a wider business both geographically and in the definition of what a "prop" is. On the other hand, I could be completely off base, and they might have sold bass-boat or airplane propellors..
At any rate, they apparently came before P & R On Frink as that operation's sign is what is still on the building. I can say with a little more certainty that this was a used car operation, though setting the closing date as 2010 is a total guess. I did not find any hit which told what the 'P' & the 'R' stood for either.