"What's the twist lever (bare aluminum) on the firewall - hand throttle?"
parking brake handle
"...and what does each do?"
The two taller ones above the black rectangular cover are the standard transmission and OD/gear splitter sticks (just like mine on the cj5)...
The two shorter ones through the round black cover are for the transfer case - its a standard D18 twin stick set-up - just like mine on the Willys, but in 1966 the D18 went to single stick and thus why my cj5 doesn't have 4 shifters. One of the twin sticks (on the left) shifts between 2-4wd, the stick on the right shifts between high and low range. So the twin stick allows for use of 2lo - which is a bit unusual these day. Best aspect of that is when in very tight cornering with manual steering (and especially with a locker), you can disengage the front axle/wheels to make steering a bit easier. Also have to remember that early jeeps were as much for farm and tractor usage, so even if 4wd might not be needed, running in low range allowed for the proper forward speed (like walking alongside it). The rear most two sticks are PTO levers - one for the front and one for the rear. Could be used to operate winches - possibly on the front - but most likely the rear one is for farm or heavy equipment operation just like a tractor.
Having both PTO and the OD is pretty rare as the OD runs off the PTO connection on the D18....thus the PTO must then be placed behind the OD...I've only seen it in pictures, but yes, my OD does have the removeable rear cover that would allow a PTO to attach. Most people using PTOs for agri work didn't have much need for the OD as it was more for road driving. I always feel like my cj5 is a bit short sticked with only 3...most early cjs have 4. If I add the OD to the Willys, then at least that one will have 4.