US M1917.

You guys nailed the details, but a quick check is how the strap is attached. A Split pin is British, a rivet is US. Not sure about -Canada?

Canada never manufactured the MkI helmet during WWI only used British stock.

This is US M1917 helmet. Z is the steel supplier and D is the company that stamped the shell.

Well I'll be damned, is there any way to tell the branch?

Doughboy is American.