Mystery tool

Picked this up cheap, but not sure what it is.

Clearly, it's some form of screwdriver -- but it has a pair of sleeves or collars and the shaft. The collars are screw-threaded, such that as you turn each one it progresses up, or down, the shaft. You turn one, then turn the other to follow it until they're snugged up together; this locks them in place.

Maybe the collar keeps the screw head from falling off the side of the screwdriver blade? I have an e-mail out to the OldTools list, so hopefully someone will have an answer.


--GG