Flatty gearbox help

Hi guys, I have a gearbox from a `50 up Merc, its a 3 speed with overdrive. I think these were also used in similar year Fords.
Thing is there are a few bits i'm not sure about with it. I have run one before in a '56 F100 and it worked well although it was already fitted when I bought the truck so I did'nt get too inolved with it.

Obviously I know where the 1st/rev and 2nd/3rd levers are but what does the other lever do further back, the one hanging down behind the o/d solenoid?
Also just a bit further back theres a small 2 bolt mounting flange with nothing in it, what should be mounted here? And on the other side of the box another gizmo doodah which I have no idea what it is?
One last little problem, the 1st/reverse lever is stuck solid, I know when the overdrive is selected this happens as there is no such thing as overdrive 1st/rev. So does that mean overdrive is on without power to the solenoid and it goes off when its energised? Oh and is it likely to be a 6or12v solenoid? Will it matter if I put 12v to it anyway?
Phew, thats it, comments appreciated. Cheers, Kerry, originalshrs@btinternet.com

 

It would be 6 volt, but I have used it with 12 with no problems. The shift lever on the tailhousing is a overdrive lockout. I think there is an internal rod that makes you turn the od off with the lever before you shift into reverse. When the od is shifted on, it is freewheeling. With if off, it is a regular 3 sp. The round thing on the pasenger side is a governor, the solinoid on the driver's side is the kick down solenoid, passing gear. You could just hood the solenoid up to a toggle switch and put a pull cable on the od shift and it would run.