Gordon:
Pending the arrival of your photographs all I can offer is educated speculation. In general, I think Dr. Rob will prove to be mostly right.
England entered World War One in 1914, and shortly thereafter started ordering Colt New Service revolvers chambered in .455 Webley, which was their service cartridge. Shipments continued until 1917, when Uncle Sam issued contracts for the 1917 model in .45ACP.
Most if not all of the British contract .455 revolvers fell in a range from 65,000 to 139,000, and your revolver would fall in this range. I think the marks you are looking at are the buffed-out remains of U.K. military property marks.
After world War Two these revolvers were obsolete, and during the late 1950’s and early 60’s many were exported to the United States as military surplus.
At the time, Colt and others still had pre-war parts, including barrels and cylinders in .45 Colt, 45 ACP, .44-40, .38 Special and of course .357 Magnum. Many people who wanted an economical .357 Magnum purchased a surplus New Service and had the barrel and cylinder changed to their caliber-of-choice. Considering that the original revolver in excellent condition cost under $20.00, and a new barrel and cylinder ran slightly more this made a lot of sense.
If I am correct I would point out that the frames that Colt used to make .357 Magnums, starting in about 1936, were stronger then those made during the 1914-17 era. Consequently I would recommend that it would probably be a good idea to stick to loads in the .38 Special +P or under range. No, you won’t blow the gun up with Magnums (or it’s unlikely you will) but you could stretch the frame
