I am now thinking that they could be used to unscrew metal gallon can screw tops
On closer inspection, the circular jaws have been welded onto a pair of (sort of) standard Buck&Hickman “cheap” pliers
As an observation, with the jaws like they are, it would be quite an unnatural motion of the hand to use them for unscrewing, especially if the top being unscrewed was on the top surface of the container (i.e. hand and arm vertical). It would involve less muscle contortion if the cap being unscrewed were on the side (hand an arm more horizontal).
Maybe have another flick through the catalogues and see if you can identify the handle shape, ignoring the business end. Even without the welded-on jaws, the straight legs are quite far apart for a normal pair of pliers.
To me, they look like a miniature version of a blacksmith’s bolt tongs, but blacksmiths ones have curved legs to accommodate the head of the bolt between jaws and pivot. Is there anything you might hold in them and then work on the other end of it?