What kind of light fixture would have been here?!

[1918ColonialRevival]

If it's regular knob & tube wiring then probably an outlet. In my childhood home there were electrical outlets in the floor which had screw-in plug sockets. I could unscrew them & screw in light bulbs! This fits with Phil's radio plug. I suppose the K&T installation didn't require electrical boxes because the lath & plaster allowed solid attachment of fixtures without the need for a box. Fire due to overloading of the circuit probably wasn't much of a concern because all one had to plug in was a lamp or a radio for the most part.

+1

If it were a call button, you would likely have bell wire behind it.

Some K&T installations didn't use boxes, but based on what I've seen, that was the exception rather than the rule and the ones I've seen have looked like old homeowner additions.

[phil]

I think they had better copper and didn't scrimp on the guage. from what I see my house wasn't underwired in this way especially considering the panel size. the thing to watch for in K and t is lumps , they will appear like a lump of black tape and I have seen it in my house, my sisters house and my brothers house and all were real fire hazards. they would just tie the wires together and tape them up. the place to really look is between the incoming power and the meter as they were often stealing power. It wasn't the electricians it was subequent contractors and homeowners that werent' careful. If they soldered them it's much safer, still not to code though. i thought my wiring was pretty safe, just old but when I started pulling wire out I found enough of this that I was really glad I rewired.
you could probably do resistance checks if you wanted.
my brother re-wired and found he had one plug before the meter so anything plugged in there was free

[Neighmond]

Looks like they had something on the mantle plugged in.