It's a bus as in Front Side Bus, it's a main artirial highway for electricity to run down, normal multicore wire would glow white hot or just melt explosivly!
. I did'nt know telephone exchanges used it though?...
That probably is solder, though I could'nt imagine trusting that kinda current to a soldered connection!, bolt holes only please!...
PS, the discolouring is just tarnish/oxidisation...