I think you guys are right. You can see the lugs on the side to hold it on top of the jaws. I’ve got one about that size and also a smaller one. My big one is cruder than the one pictured but is also made of 3/4 inch stock.
The smaller one is made of 1/2 inch square stock and is called a "vise jockey". It was patented by Carl Petersen on May 6, 1952, no. 2,595,699 and looks pretty much like the patent drawing.
Mike