Henri,while "surplus 1917s,especially the Colts,more so than the S&W versions were used and even U.S.P.O. marked,the ones I am most familiar with are the Bankers Specials. These were issued to the Railway Mail Service clerks in the 1930's-1960's. These were the men who sorted mail in Railway Post Office Cars,on passenger trains,and there was still a fear of Robbery!!. 1917's had been tried in the 20's,but they were just too bulky to be worn in the narrow confines of the cars.
The Bankers Specials were .38s(not .38 Specials),and later I believe some Detective Specials were used,along with Police Positives by the clerks.(S&W made about 500 2" .38 Regulation Polices,5 shots,for the R.M.S. div. of the U.S.P.O.).
These are the only Colts that I know of that were "Official Issue",by the P.O.-but may private guns were carried,especially on those railroads,that were too small to have a full time RPO car and clerk;these men usually had duties for mail,expess,and baggage,earning only part of their pay from the railroad,the rest by the express firms and P.O.
I sold a U.S.P.O. marked holster,for a Bankers Special on another forum. It had last been used by a Mail Truck driver in the 60's. Bud