deSoto:
I remember those. They were still a semi-common sight in some of the apartments I lived in as a kid in the 1980s (believe it or not).
I believe the casing is a hard rubber material, like what's used for plugs. Not bakelite plastic.
Somewhere I have one at home also from that same exact company.
We've gone full circle it seems. Current-manufacture screw-plug fuses (from Buss) are built inside white plastic housings. These used to be ceramic when I was younger.
