(returning from my coffin hunt)
Ok, most often this box was used if there was an "add on" to an old service that was 220v without a neutral, such as a water heater. Yes, if the white is hot, it should be painted or have marker tape on it, but I can't remember ever seeing any that were! There is no neutral block in this box, just what looks like a ground wire. To make things worse, the power input (or what should be the power input) does not even have a ground! It even looks like the primary wiring is that funky old cloth covered rubber romex that was used in the 1940s! (I hope I'm wrong. I used to call that stuff "bomb wick" as it often ended in a fire.)
Now, you mentioned an FEL, so if this is feeding a device that is 120v, the "neutral" is running through the fuse that was intended for the second hot. I haven't seen something this far from code since Frankenstein's Castle! (ahh.. correction there, the Playpen in 1980, but that's another story.
Best way to address this box: Wrap yellow "Caution" tape around it and RUN !
From the pictures, the fuses look to be different, perhaps originally it was designated for 30A, but when they wired to L5-15R they switched the hot fuse to 15A, leaving the neutral at 30A. The line has no ground, but the L5-15R is grounded to the inside of the disconnect, as you can see.
Edit: Perhaps this is a case for my DMM, but I think ETCP certified Derek Leffew might discourage me from sticking probes in there.
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