Chris Lewis wrote:

....
How many houses had any dimming whatsoever prior to SCR/triac dimmers?
How many houses had more than one? How many of these were used
in general purpose things rather than very specific (and usually
low wattage) fixtures?

I would submit that up until the late 60's, general purpose dimming
circuits were almost non-existant in residences. What dimming
circuits there were were expensive heavy-duty industrial units,
or low wattage (usually considerably less than 50W), often a part
of the fixture itself, or the cabinet it was installed in.
....
In 1963 my family moved into a (new) house that had a dimmer
switch for the dining room ceiling light. While it was about
the same size as today's dimmers, it was not today's 4 component
circuit. I remember looking at the schematic. There were a dozen
or more components, including a transformer. I didn't know
enough then to understand the circuit, so I can't offer more details.