Porcilin 4/O - never seen one of those.... And the NM cable looks to be about right for 50's previously it would be rubber with string/twine insulation, kraft paper, cambric loom, and lead paint.... late 1940's post WW2. Thermoplastic would be 50's, and plastic jacket would be late 50's
On the topic of splices: The largest soldered splice I have ever seen was 5-500MCM taps to DC disconnects for four elevators. Still live DC service and metering... The building was pre-1906, and no-one knew how old it actually was - a fire after an earthquake here destroyed all records - but the elevators were some of the first Otis models. These taps were not in an enclosure and trussed up in a room as a buss from the service conductors. I had the pleasure of unwrapping and re-wrapping them to see if they were still intact - they were... Each conductor was splayed out, then lashed in on top of the others re-wound into one really lage wire, then an other outer lashing - then soldered and taped. It must have taken all day to heat up the mass of copper without burning it, and still get it hot enough for the solder to flow.... Maybe laddled the solder over and over... A dead art IMO... This was way before digital cameras - and wish I could go do it again just to get a picture this time....