The first Colt Model 1917 revolvers did not have a shoulder in the cylinder, and the .45ACP they were made for would fall down into the cylinder. They were intended to be used with the half moon clips. After WWI the .45 Auto Rim cartridge was developed so that the pistol could be fired without the clips.
You can bet your paycheck that the Britishdidn't alter the pistol. Some of the Colt SAA's they bought were 7 1/2 in barrel, and they were not altered. All they wanted was pistols, and customizing them was the last thing on their mind. The cutting, drilling, and stamping was done after it was sold by the British government.
What are the markings under the crown which is under the broad-arrow property mark?