The inner helmet was known as a G-Helmet and was common on all types. On aircraft without pressure breathing it was worn with an H-mask. Pressure breathing masks were originally the M-mask but this could not hold high pressure to the P (large face - bridge of nose to chin) and Q (medium face) were introduced.
It was originally thought that rear crew did not need the sun visor as they didn't look out (economy measure?) but by the early '60s visors all new bone domes were fitted with a visor.
We used to wear the bone dome take-off and landing and discard it as soon as we could. Later there was more acceptance of wearing it while low level flying.
Fubaar has a point which was why there was early reluctance to wearing the bone dome all the time. I think attitudes changed as the bone dome did have its uses in low level turbulence and a planned low level abandonment was not necessarily a slow process.
I was fortunate in never having to abandon an aircraft but we did a test once, Irish Sea, 500 feet 250 kt, all engines throttled to flight idle and a climb commenced. We topped at 1500 feet so there was some useful time in the event of loss of all 4-engines - say an extra 500 feet.
Of the low level crashes though I don't think they were survivable. The Shiraz crash on landing was a good example for wearing a bone dome, although I don't know if it was a factor in the survival of the rear crew.