Last night I was riding as it was getting dark, when a guy on some modern noisy bike came up alongside at the lights and shouted that I had no rear light. I stopped when I could and had a fiddle around, I could see nothing obvious, but I noticed that there was no brake light either, so straight away I knew I had an earth problem.
Today I took the rear light assembly off, and sure enough the metal plate at the outside end of the lamp assembly was adrift, held there just by the earth wire. I can't be sure if the thing was once crimped on somehow, but this had been glued there, and this had come unstuck. Looking at the parts book, the whole thing, apart from the lens, screws and bulb, you've guessed it, is one assembly, and a search reveals it to be NLA. I did find a complete K0/K1 pattern tailamp available new from Japan, but with the usual horrors of postage, duty etc to add on.
For now it has had a bigger blob of fresh araldite to stick it back together, and is sandwiched in the vice between a socket and the clutch tool, to squeeze it together while the araldite sets. For now the bulb holder is making electrical contact with the metal plate with the earth tag on. I'll leave it there till Sunday, then see how its worked out.
If its no good I'll be hunting for another rear light assembly. My General Export Type lens is a huge lunchbox of a thing, with 4 screws, like the later K2 type, and I'd like to keep it original. The assembly has no common parts with the 2 screw type apart from the bulb, different reflector, everything. However, I may not find another and have to use a 2 screw K0/K1 assembly instead, we'll see.