This came up during some work on a Marshall 1960A cab last week. Had the whole thing apart, and upon reassembly and testing found one that slipped by QA. For background, these cabs are selectable for either dual 8 ohm stereo, or mono in 4 or 16 ohm. The readings checked out correct for each side in stereo, but I got 8 or 16 in mono. After some tracing, I got out the magnifier goggles and found what's in the below pic. One set of pins for a jack was never soldered to the board 21 years ago. Soldered it, and it tested correct. This cab had been running on two speakers when 4 ohm mode, which would have halved its power rating.