I have to add that a lot of cable outers both brake and derailleur in this
period were simple steel coloured coil spring steel with no covering at all.
I found a good source of this stuff in stainless steel. Try asking your
local garage welder or mechanic for the old MIG welder conduit. This is the
cable that carries gas , welding current and wire to the torch. Inside is a
3 metre plus length of s/steel coil tube about 4mm dia. There may well be
other commercial sources for this stuff. Its perfect for brakes, a bit heavy
for derailleur. Wont rust either
Mark Stevens --Spring has sprung in Dingwall Scotland.