Dead front fuse block. I still see that type of pull-out fuse holder used in some medium voltage switchgear. I'm kind of surprised you've never seen that before.
I had a panel where the screw in fuses used fuse holders that plugged into the panel. That way, the number of 120V vs 240V circuits served by the panel can be changed simply by using different fuse holders. It was a slick idea compared to a fixed number of 120V and 240V circuits like that panel has.
This is holder for 120V screw in fuse circuits. It stays in the panel once installed.
And this holder is for 30A, 240V circuits. It has to be removed to unscrew the fuses so it forces the disconnecting of both phases.
The panel had a single big 60A pull-out block for the stove wire.