On Wed, 25 Apr 2018 18:10:14 -0400, Ralph Mowery
wrote:

Many places had one side of an outlet switched. It was often before
overhead lighting. The side that was switched would have a lamp plugged
into it.

Actually you have that backward. Overhead lighting was standard long
before switched receptacles became the norm. The oldest lights were
converted from gas. That is why all of the standard luminaire parts
use pipe thread sizes although they are running threads now, not
taper. I worked on a project in Sarasota where they were restoring
some old chandeliers from the 10s and 20s that did have tapered
threads in them. It made using modern parts hard so they chased the
threads with a running thread tap.