We think our house was built in the 1920's. The walls are solid (not
cored) cement block. Replacing a cellar window, I got looking at the
lintel over the opening. The surface is cementitious, but smooth, like
a casting. (The wall blocks have a pronounced raised texture.) When I
drilled into the bottom for a (Tapcon) screw, it seemed to be at least
partly hollow.

I'd appreciate any information (or even informed speculation) on what a
lintel from that period might be made of.

Thanks,
George