It is a WW2 RCAF summer flying helmet. The snap on top was for the associated externally-mounted wiring loom. This particular helmet is referenced on pages 70 and 226 of Mick Prodger's "Vintage Flying Helmets". According to my research, these were not worn overseas and were only worn in Canada.
Here are a couple of pictures of the example in my collection (just recently sold), showing the oxygen mask as well as the correct wiring loom configuration:
In my example, the wiring loom was connected to the microphone on a mounted in an RCAF Gutta Percha oxygen mask. The only thing non-original on mine was the chinstrap, as the original was missing.
It was a beautiful setup - and one of the nicest flying helmets in my collection. I somewhat regret selling it, but I needed to raise some money for some WW1 items that I coveted even more!