Hello Russ,
the letter "R" behind the individual number of the gun has been seen once in a while, not only on guns, but also on bayonets and other equipment. There is no eplanation of the "R" at the end in the official marking orders for prussia from 1922 or 1932.
It could be that it means "Revier" (=precinct) as Jan said, but another guess is, that it means "Reserve" (=reserve weapon). Usually the police keeps some additional weapons stored away to issue them only when needed or while some officers weapons where in repair or maintenance. This could explain the "X" over the "R", meaning that the weapon went from reserve status to officially issued status, when it has been taken out of the reserve pool permanently.
But this is so far just another guess.
Best regards, Andy