Thanks all for an interesting discussion - not quite the full answer yet, I feel - what I'd love is to hear from some one who used such a vice/cramp/clamp/chops, these are obviously not just a one off, so there must be reasons for the puzzling details.
So - I promised some pix - firstly 3 of the bigger one (99p, sale price and he was surprised when I said I couldn't take it for less than a tenner), which like the first example , looks commercially made
As you see, it's pretty big, dwarfing the 9in Parkinson quick release set into the low, sit-on bench I keep outside - in fact bigger than some bench leg vices, and could easily be adapted as such, but again it has the single big bolt. no other obvious means of fixation, and those steel plated jaws with notch and steel plating at the other end. Note - I file tested the steel, it is fairly soft, so not an engineering-type use. When I said the 'commercial' versions were identical, I hadn't looked closely enough, they share the basic design and the puzzling (to me) single fixing bolt hole and plated jaws with slot, but have slight differences in size, shape etc.
In the second and third pix I show how it could possibly be mounted, still operate the tommy bar, and be useful for detailed work by swivelling on the single bolt (probably through a block of wood), eg for detail spokeshaving or carving - then maybe the slots would hold in position whatever block you'd make to hold the workpiece. but I have no evidence it was used like this.
Then this pic shows a smaller (user made? but well made, example).
Again the single bolt hole (again came with bolt and running through a block of wood) but in this case I now see that the bolt is on what I'd call the moving jaw - my mounting suggestions wouldn't work so well here.
I have looked again in various books etc, the nearest I got was in Percy Blandford's book on country crafts, which showed a wheelwrights vice but it was wider than these and standing vertically from the middle of a very low bench - not a very clear drawing.
ps thanks blister - I think the staggered holes and pin on chain are probably quite well-known to the above contributors - this is an arrangement to keep the jaws more or less parallel if required, still available today on some (v expensive) wooden leg vices.