Hi Jake yes it is a rimless helmet and IMO they are quite rare indeed I do not believe very many examples have survived and I can honestly say I only know of one other in a private collection here in Canada that I had the opportunity to handle quite a few years back.
I think they are under appreciated and represents a very important period in Canadian history and would not be surprised if a good many were left behind at Dunkirk and or stripped and discarded for scrap steel when the new Mk II helmets became available in late 1940.
And I guess if it turns out to be the British variation I could find a little room somewhere in my collection for it Jeez !!! with any luck we will get a better look at it in a week or so.