Finally had a chance to crack open that other box. Found a small fuse panel with one 30a fuse in place. The wires leaving the fuse appear to be #10, but I can't say for sure. Also, if I didn't mention it before, the right hand box has #6 wires exiting through the top that bypass the left fuse box altogether.
There doesn't appear to be another junction before the wires head up to the attic. I tried to track the wires once in the attic, but it's an absolute rat's nest of mixed types--knob and tube, metal conduit, older sheathed cable, and Romex. I now doubt I'll be able to find where (or if) the wires step down properly and safely to #12, so have to assume they're at least on that 30a fuse, or worse yet on that 40a breaker.
We've never had a hint of electrical problems in our 9 years here, but this is obviously a bit worrisome. I'll be contacting a pro soon to get to the bottom of this thing.