Flatcat,
as already mentioned by Jim, RFV stands for Reichsfinanzverwaltung. To this authority also belonged the customs services (Zoll in German). To these officers your Sauer 1919 did belong.
BTW: some collectors do use the term "model 1919" for the smaller version of the 1913. It is assumed, that the model 1919 in .25 cal. came out in 1919.
It isn't easy to make a statement, when your pistol was made. Based on serial number studies I'd say approximately (!) in 1921.
Your's is a first variation with an added "Zusatzsicherung" (additional safety), at least, I bet it has a Zusatzsicherung. All Sauers of the police and of the Zoll were amended in the twenties and in the thirties by adding the Zusatzsicherung. This is a small spring arranged oblique to the safety lever. Check out this photo, where I explained this:
http://luger.gunboards.com/uploaded/...26_30600-3.jpg
Normally, the first variation (SN 1 to 30,000 app.) don't have the Zusatzsicherung, except for the police and Zoll variations. Regularly, the Zusatzsicherung started with the second variation at SN 30,000.
BTW: a friend of mine is currently writing a book about German police markings of the Weimar period. He also lists all known RFV pistols. For this purpose, the full SN would be helpful and I'd appreciate it, when you would tell me the SN.
Best regards
Martin