WW2 BRITISH “M” TOMMY HELMET ( MESSENGER ) SGT. ?

I'm presuming this is motorcycle Messenger.
If anyone knows different , please enlighten me as I can't find that particular designation in my books or online.
Rare piece.
Large " M " and three chevrons to front.
The insignia is commensurate and all original to it.
Black painted shell.
Good liner and chin strap.
1938 date to liner strap.
V.G.C.

Code: 59722