In Austria at that time you'd have most likely found a wood frame with open back plastered into the brick wall, containing a thick bunch of all black cloth wires, twisted and taped together. Until the advent of PVC boxes and PVC insulated wires in the 1960s (though cheapskates continued using old metal boxes and conduit well into thee mid 1960s) that was the only wiring type used in construction.
To improve the dielectric properties of the cotton covering that stuff was drenched with tar (at least that's what it looks like), turning everything in a black mess. There's hardly anything that gives you worse black hands than that!
Well, the old cloth electrical tape is similar, but also sticky...
