All the rust, corrosion and debris in that panel says a Sparky needs to evaluate and make all necessary repairs anyway, so just make a general statement and hand it off!
I do not know the answer to the question. But that neutral lug on the first panel looks like an “add-on” or modification. (and it is rusted)
I am with you, I would like to know the answer. Four hots and one neutral? What happened to the other neutral? Apparently they split the hot feed and not the neutral. Weird.
From Oklahoma, it looks like there is a Neutral Jumper going from the first panel (top pic) to the second panel (bottom pic).
It also appears that the 2 hots for the second panel are traveling thru the first panel, if so, panels are not allowed to be used as a chase.
The neutral jumper wouldnt make it any different than any other distribution panel setup. The only difference is there isnt breaker for the hot conductors, so the distribution panel now has 7 hand movements. I am not absolutely sure, but it seems like that, along with what David said about the chase would be the only issues. Oh, and the lack of a 4th conductor (grounding), unless that is a metal raceway bonding the two panels together.