These are traditional pattern makers core box clamps.
They are sold in pairs and consist of six pieces of steel. The two outside pieces are screwed onto either side of the core box centre line. The central piece is then pushed onto each dovetail slide pulling the boxes tightly together and clamping them firmly prior to ramming in the sand.
The central clamping slide has a 5mm hole for easing loose with a pull bar.
Another use for the core box clamps in the small foundry is to clamp the cope and drag together on wooden moulding boxes. Dowels are normally used for allignment purposes