I run into similar joints when I am working on old railroad cars.

The heavy old wire was originally for a 32 volt dc system, low voltage, high current, big wire.

At some point along the line, somebody converted the whole shebang to 110, or 240 volt ac. Higher voltage, lower current, don't need such big wires. Big wires can carry low current, they just don't get as hot the rubber insulation lasts longer. [Linked Image]

I'd bet a fried chicken tv dinner that's what happened here.

TW