On Marines guarding the US Mail, contemporary photos show them armed with the M1911 pistol, shotguns and BARs. I have not detected the use of any Colt or S&W Model of 1917 revolvers in that role. It is certainly possible but I have not seen any photo evidence.
Of course, the US Post Office's use of the Model of 1917 revolvers is well documented, but here we are talking about Marine Corps weapons. I tend to doubt that the Marines would want to rely on the USPO to supply the Corps with any weapons.
The link to the 2001 Museum article was interesting but in my view not dispositive of the question about USMC-marked M1917 revolvers used by those guarding the mails. Careful reading of the article reveals that the two revolvers in the Marine Corps Museum at the time were both donated to the Museum
in 2000 by a former Marine. There is no evidence cited that the former Marine was himself a mail guard. Figure that a 25 year old Marine in 1925 would be 100 years old in 2000. Not impossible but not likely either.
The article goes on to claim, rather inexplicably, that "
The Army revolver has exceptional provenance that now give the Museum a 'real' tie to the Marines' mail-guard mission." Just what that "provenance" is remains unstated. I would not rely on that article to prove anything other than the fact that two revolvers were gifted by a former Marine in 2000.
One possibility is that the M1917 was marked after WW2, not beforehand. During that post-war period up until about 1960 or so there are many examples of small arms that were marked Marine Corps Property. Examples I know of include the Colt Woodsman pistols shipped in 1952 with factory USMC markings; Colt Officer's Model revolvers shipped to the Marines in the late '40s with post-factory property markings; and the Harrington & Richardson MC-58 .22LR rifles shipped in the '50s with factory applied property markings. There are other examples as well. The Marines seem to have gone through a phase of marking mania during that time frame. Thus, while the M1917 was obsolete by the end of WW2 I would not rule out the possibility of post-war marking.
One other small point occurs to me. The Gunbroker M1917 was marked PROPERTY OF USMC while the example shown in the Museum article was marked USMC PROPERTY. That may be of no significance but does demonstrate a certain lack of uniformity.
Regards,
Charlie