Edit: the following may be speculation... I misread the linked post. R before the serial is for "replacement", so the bridge was replaced during service. Source: https://omegaforums.net/threads/speedmaster-with-strange-serial-number.25969/#post-288118
Reading through that thread, it does seem to be the consensus that at least the bridge was replaced, maybe even the whole movement. But it's the correct caliber. If that's truly what's going on, that wouldn't bother me at all, personally.
Another thing I should add is that the second hand doesn't stop moving when I pull out the crown to adjust the time... I thought this was a sure sign it was fake. Am I wrong?
The movement in this watch does not hack, so that is not a problem. I don't have an explanation for the 5-digit serial number proceeded by "R". It's unusual and I'm not familiar with it. A lot can happen to a watch in 70+ years, and we are always learning.
Seems like you got a nice Omega 2848 with a replacement crown and some movement parts that may have been replaced. Signs that the watch was cared for by previous owners during the last 60 years. I think it's time to stop questioning the authenticity and just enjoy the watch. I wish my wife would surprise me with something like this...