Not sure if this will wind up in Electrical Nostalgia or just Pics Submitted for discussion - I leave it to your discretion!
Found this one while I was doing some pre-sale "code compliance" work around a former police officer's house. You know, GFCI and smoke detector installation type stuff. Seems the guy he bought the house from 20+ years ago was obtaining things from somewhere when he built the house in the 50's. The whole garage was done in a hybrid of RMC and EMT, with mega-godzilla quality fittings. It's got a real "piece from here, piece from there" feel - very few parts of this garage's conduitry seemed "made-to-order".
The best I can figure, this is a Crouse - Hinds Explosion Proof (or really raintight) Edison base fitting. Not sure if it's for a fuse, or if it's a power tap of sorts. Yes, it is gasketed, and yes, I did manage to liberate it for the collection!
Picture 1 is with the cover closed, and pic 2 is with it open.
Sorry for the low-res... it's a camera phone.
Have a good weekend,