Thousands of Mk. II*** rifles were supplied to the US in 1917. Some had their barrels set back. The shank in front of the receiver ring is shorter than if the barrel had not been altered.

The thumbpiece is riveted to the magazine follower. Not unusual for it to be removed when a rifle was sporterized. Replacing the magazine follower w/thumbpiece is easy enough to do.
When made, the rifle had a magazine cutoff in the front of the trigger guard and a spring. Can't see it in your photos.