Hi all ...
I believe the original Mk1 "outer" bone dome was issued without a visor but allowed for the wearing of flying goggles (the two male popper-studs at the rear of the helmet were originally designed to take elasticated webbing to keep the goggles strap in place). The picture below of a Canberra crew from the early 50's shows the Observer wearing the Mk1 "outer" without goggles and the Pilot wearing the electric hat "inner" plus goggles.
I believe the Mk1a "outer" was then introduced with a moulded forehead arrangement on the bone dome that allowed for the fixing of a track and visor as required. The picture below (again from the 50's) shows a Victor Bomb-Aimer with what looks like a Mk1a but with no track/visor arrangement.
The final picture from the Mid 60's shows Nav students in the back of a Dominie resplendent in their Mk1a's plus visors.
I recall that a mod was introduced to file-down the central plastic visor actuator to make it less unicornesk given a spate of cracked perspex canopies on the Chipmunk TMk 10 encountered during aerobatics or turbulence
Sorry if a bit off thread ... but thought it might be of interest !
Best regards and Happy Christmas to everyone...
Coff.