I don't think the neutral is supposed to be fused lol. Not that it really matters. Looks like this can control either two 120V circuits or one 240V? I have an NOA Cutler-Hammer one from the 70s or 80s like this but it uses two 30A cartridge fuses.
Yeah, I guess they had two fuseholders so it can used for 240v loads like Mike said. They still sell these fused disconnects here too along with the fused inserts for old fuse panels.