I think the USMC version had a narrower butt than Army and Navy versions. Or, maybe that was just with the preceeding .38's?

The M-1917 (not 1918) Colt is the same size as the M-1909, and is basically the same gun, other than for caliber. The extractor rod head on the 1909 model had three lobes, and many M-1917's had only two, as did other later New Service Colts.

The M-1909 takes a special version of the .45 Colt, with a wider rim, to give more certain extracton in a swingout cylinder.

Finally, the 1909 had a conventional blue finish, while many 1917's had a dull gray finish.

Neither gun will fire the cartridge of the other, although power level is similar.

Frankford Arsenal loaded the special M-1909 ammo. I don't think it was ever made commercially, although I could be wrong.