Picked this up at a yard sale yesterday. Can't see any marks on it. Maybe after I clean it up. Any ideas on who the maker was? Looks to have a 2 jaw system. Anyone know what the thumb screw on the side is for? 

If there is a mark, its likely to be very fine stampings on the side of the chuck.   I've not seen a thumbscrew in that position.   But the fact it has ball bearings tells me it was a higher end tool.   Nice long chain too.   

I am guessing the thumbscrew locks the shaft so you can unscrew the chuck....
Otherwise you would be trying to hold on to the thin threaded shaft if you didn't have a brace attached (Which would be somewhat awkward)

The chuck somewhat resembles a GP model, but the rest is different, unless it is a very old one model....

It is a nice clean design in any case, and looks well made....