I don't think I've ever encountered a plug-in meter with a luminous face, they've all been standard black paint on a white background. The 'three part glass' in the meter protective cover is almost certainly a piece of laminated glass that has delaminated with age. The outer sheets of glass are for scratch resistance and the inner plastic/adhesive layer is to prevent splintering if it gets broken. (Glass either side because you won't know which way it gets put in.)
Meters with plastic (plexiglas/perspex or celluloid) lenses were a later (cheaper) approach, and they discolour and distort (celluloid) or suffer from crazing (plexiglas).
The meter protector is to keep dirt and damp out of the equipment while still allowing you to change the meter if it fails - and I think they issued spare meters (and designed them to be plug-in) for that reason. (It also allows them to be tested easily during overhaul, without dismantling the equipment)
Chris.
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