Another oddball found while unpacking and inventorying my collection tonight. It's a fuse holder insulator that's 4" tall, 3-1/8" wide, and designed (apparently) to thread onto a pin sticking out the bottom of the crossarm. The threads match up with those on my CD 590s and it was in a box with a bunch of Australian stuff, so I believe I picked it up on my AU/NZ trip with my father back in 1991.