Now here's a useful post on 68kmla, which also solves one of my curious questions.

When I mostly dismantled my Duo 230, one of the things I found odd was the lack of capacitors. Admittingly, while most of the interesting stuff on motherboards this side of my birthdate are implemented by ICs, it is kind of hard to make such a logic board without some capacitors :P.

Looks like they're mostly clustered near the DC power input socket and the serial port. Which is located underneath the northern end of the frame, where the hinge mounts. The square piece, looks like the serial port behind the flip down leg next to the power socket. Despite going far enough to remove the display panel, I didn't take that much of the frame apart in my efforts to inspect the insides. Honestly, I was just surprised by the lack of plastic disintegrating the moment I unscrewed the damned thing 🤣.