A flat tab?  Others I have seen use wooden ones, but these were used by blacksmith's and foundrymen too,
could they have had a removable so the wood would not burn?  maybe a T-handle style?

What is the text on the base of the movable jaw?  Maker name & model number?  Patent date?

The missing crank handle most likely was like those on "Pony" clamps. ( http://www.ebay.com/bhp/pony-clamp ) has photos of several styles of "Pony" clamps.

If That's a Cincinnati Tool Clamp it should look like this http://www.ebay.com/itm/Vintage-Hargrave-Cin-Co-Bar-Clamp-and-Brink-Cotton-4-C-clamp-144-2-Total-/221857983612?hash=item33a7c42c7c

yes that is it Cinncinnati tool co. #506-a  [size=78%]Hargrave patent pending, circle H on each casting,[/size]
[/size][size=78%] so that would be Hargrave's logo?[/size]


Now to follow the Master Uri Hofi has shown how to make "Mickey Ear nuts"
oh he does make it look easy too!   think I will give it a shot. "some guy in Illinois" <grin>

1st attempt at a  wingnut, not bad but not that good.
Started out as a pc of 1" wide 1/4" thick 3" long

If you decide not to use it as a wingnut, you could use it as a bowtie for a robot.  Perhaps it could placate our interstellar overlords....