Perhaps the single post-WW2 Nitro W German proof mark might possibly mean a repaired firearm that required (by law) the proof be done.
Normally though there is the set of 3 marks on the frame,,the particular Proof House,,the yr/date code,,and the Nitro Proof mark.
The bbl and cylinder recv the Nitro Proof marking alone usually (but not always).
There is (was?) also a marking to indicate a 'repair proof',,but I honestly don't know if it's used on non-German mfg arms or not.
Seems like a repair (re)proof would have caught the fact that it hadn't recv'd the Gov't demanded initial proofing in the first place and the entire thing would have been done as described by 'usnrigger'.
Anything sold on the civilian market has to be Gov't proofed and marked.
Another thought is that the bbl alone is a replacement to the gun that just happens to have come off of another revolver that had been full proofed in W Germany.
Just some thoughts,,,