Originally Posted by
dsbrantjr
Some rear derailleur hangers take some space in the rear of the right dropout. Perhaps your spacer was inserted when such a derailleur was present to take up a similar amount of space so that the wheel would sit straight when pulled all the way back on both sides.
My thoughts initially, but all bolt-on hangers work the same way, so if he removed it, how/where would the OP attach the RD. Based on that I suspect that he had a "simplex" or similar wheel stop in a conventional dropout. These were used in pairs to serve the same purpose as the micro-adjust screws that Campagnolo popularized.