A good condition Siebe Gorman 12 bolt diving helmet.
Detachable front face plate, side windows with guards, front hanging posts, lashing eyes to the top, side hooks, spitcock, rear air inlet and pepper pot. Never had comms. 
Not matching numbers, the corselet is 1st WW era and the bonnet 2nd WW.
No brailles as reflected in the price, but this may change if I source some!