I have one with only that mark on the heel of the butt. Shipped to the Canadian Government 1915. Perfectly correct. I don't have a pic of the mark, but trust me, it's the only mark on it.
Yes, I have seen that mark on other Canadian .455 HE's too. Just nitpicking, I do not believe it is an "import mark", though, but a military acceptance stamp.
I believe the other stampings your potential customer is looking for are the post-service commercial proof marks. The absence of those just indicates the gun left Canadian service in some "unofficial" manner; likely an officer took it home.
The mark is the Canadian military acceptance/inspector stamp. The gun is 100% correct. It went to Canada, not England, and would not have had any British stampings. Your buyer is uninformed about Canadian guns. Ed.