Fascinating that yours here is 4000 SN block later than the OP, but comes without a stock lug to it in the 1908 style instead of the 1914 style with a lug. I owned a commercial exported to the US 80395 that was 1914 commercial style frame but the slightly under 4” barrel in .30 cal.

I can't answer all of your questions, but I believe that these examples represent the police Armories assembling and refurbishing P.08 pistols to supply the Post WW I Police Departments. Pictured below is a similar example of a 1908 Commercial C/N prewar pistol chamber dated 1921 with the sear safety and property marked to the Berlin Police.
I've seen the toggle pins marked with two ,three and four digits. Also, with two on each end.

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O.P.M. its a Mecklenburg stamp for a police pistol most of the examples on page 240 of History Writ in Steel show many 5 digit a suffix pistol marked 1921 that obviously didnt start their life in 1921. Here is a pic.

 

O.P.M. its a Mecklenburg stamp for a police pistol most of the examples on page 240 of History Writ in Steel show many 5 digit a suffix pistol marked 1921 that obviously didnt start their life in 1921. Here is a pic.
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Excellent points.

 

Very interesting! Doubly so in that this variation seems almost exclusive to the OPM. Much appreciated!