"Well, you could have labeled them series, series/parallel, parallel, etc. That would have been much clearer. "
We all said, in different ways, 2 pairs of 16 ohm speakers wired in parallel, would give you 8 ohm pairs for the stereo function. And, both of those pairs wired together in parallel would give you 4 ohms for the mono funtion.
LK's lower right digram shows the 2 parallel pairs wired in parallel for the 4 ohm mono function. That configuration split in 2 would give the 2 x 8 ohm pairs for the stereo function,
The good news is that unless someone has gotten inside the cab and dinked up the wiring, all that will need to be done is to replace the speakers one at a time, wiring them exactly as they were wired to start with.
Edit: I overlooked that it may be set to wire the 2 x 8 ohm pairs in series for mono, which would work for the 16 ohm mono function. I doubt they are doing it that way, but you never know.