There's no electrical detail for the 10.5 but it's half an Omni 10, so the two relevant options are
Parallel - gives higher sensitivity for a brighter sound
Series - reduces tweeter output if option 1 is too bright
My guess is that the switch is simply to cut the tweeters out. The tweeters go in parallel with the woofer, + to +, and the speakons go 1+/1- to + and -.
You could get all sophisticated and work out a way of switching between no tweeter, series tweeters, and parallel tweeters. But first you'll need to work out where the other ends of all the wires go, and maybe label them all.