The correct term is CREEPER gearbox.
I found this info from an old post by Lucky.
All right, here you go. I have one on my b7100 that is very similar. It is a creeper gear. Shifting is the same as with a b7100dt, except if you pull straight back between 1st and 2nd in either high or low, you can get into the exceptionally slow creeping speed. I think you will also find that the additional lever on the right side is for fast or slow creeping.
Mine has come in very handy for tilling, but it was originally meant for use with the trencher in which you have attached. Every once in awhile I see one of these, but not very often. It is right from the factory and not an aftermarket addition.
The hoses attached to it are to pump hydraulic fluid up to the gears in the creeper which are above the normal transmission fluid.
I do not see any recent posts by Lucky but I suggest you send him a forum email asking for any more info he has.
Dave