If it is an unaltered Colt 1905 (the full correct name would be "Colt New Army Model 1905"), than it is undoubtedly chambered for the .38 Long Colt (also known as the .38 Army, and the .38 Navy). According to Ezell's Handguns of the World, the 1905 was a U.S. Marine variant of the New Army 1894; differing only in it's rounded grip. Whoever told you that the Marines chambered it in .38 Super may be mistaking that cartridge for it's parent cartridge, the more anemic .38 ACP (not to be confused with the .38
0 ACP), which shares the same 9x23 case dimensions. Some years after your friend's gun was made, revolvers were being chambered for the .45 ACP, so I suppose it is possible that the Marines did this with the .38 ACP. I am not a scholar on this subject, but it strikes me as highly unlikely, as this was not a standard service round, and would have no ballistic advantage (and maybe a slight disadvantage) to the .38 LC. If, it did happen, however, than the breech-face of the cylinder should be rebated to accept moon-clips, or the chambers should possess a shoulder, as the rimless .38 ACP would have nowhere to headspace in a straight bored chamber. In any case, guy sajer is right; neither .38 Special nor .38 Super should be fired from it. Personally, I wouldn't shoot it at all until I knew for sure what it was chambered for.
As far as collector value, there seems to be little information on that particular revolver available online. You may have better luck looking one of the old fashioned "books" on the subject. Or you can request a history from Colt, which should run your friend a hundred bucks. Here is a link to a page on the Model 1894:
Colt New army Nodel 1894
That should help you get an idea of where to look for the various markings. Here is Colt's Archive Services page:
Archive Services