A couple of questions about my Ross

I have a 1909 dated Mk II 3* Ross which is one of the rifles sold to the US during WW1.
It is complete and, as far as I can tell, original, except that I replaced the rear top wood with a piece from another rifle when I bought both of them, 25 years ago or so.
The first question concerns the chamber. The receiver and the barrel are not marked with an LC or an E indicating an enlarged chamber. Yet, the empty cases from shooting the rifle seem to be over large in the neck area, compared with other .303 rifles I have shot. Is this normal, or is it just a case that the rifle is unmarked?
Related to this, why were chambers enlarged on the MkII, as they were not intended to be used in combat? I am probably wrong, but I was under the impression that the enlarged chambers were the result of feedback from the bad experiences of the Ross (later version) in combat.
Secondly, among the markings on my rifle is 73 CEF 712. I take that to mean that the rifle at one time belonged to the 73rd Battalion CEF , which was ( I believe) the Montreal area section of the Canadian Highlanders. Since they got to France, would that indicate that this rifle actually made it as far as the UK, and was shipped back. I understand that there were some Mk II rifles that were. There is a picture on the web which purports to be of 73 Battalion troops with MkII Ross rifles. It may have been taken in England.
A recent visit to the excellent Canadian Forces Museum in Calgary has renewed my interest in the history of this rifle and the men who may have carried it.
Any help with my questions would be appreciated.

Some Mk. II rifles were used in England for training purposes. Perhaps that is where the chamber was modified.