It is a vernier control, like might be in a utility vehicle or a gen-av airplane. I forget where I got it, think I shopped around and found it for like $35.
similiar units:
Horton Truck - Parts Detail
eBay Motors: LOCKABLE VERNIER THROTTLE CABLE ASSEMBLY (item 130132963664 end time Mar-30-09 11:36:45 PDT)
The twist fine tune function is nice, hopefully I can dial in best mpg on the flats with it.