Without doubt a fair dinkum M15! Nothing wrong with it, even the chinstrap seems original. It seems though that the strap in a few spots begins to show some white mould, but that may be due to the pic rather than reality. However, if it's beginning mould, you might want to treat it with alcohol which is a fine mould killer. I know: my stomach may have ulcers, but definitely not mould
In short: you've got a good helmet there!