I don't recognize this item, but have a couple of ideas that may help someone else. First, observe that the large "clamp" feature has no fixed feature to clamp against. This makes me think that the small jaw is used first, gripping the end* of something like a belt or tape, and that the half-round notch is then set against a round item such as a pipe, adjusted, and then the tool used to stretch the part being gripped. Imagine an upholsterer stretching fabric tapes for a seat, for example If the adjustment were near-perfect, this might even sort of toggle, making it easy to hold tension while driving staples or tacks. After attaching the tape, the tool could be removed, and the tape cut off, ready to be part of the next strip.
*Gripping a fold would mean that there would be no wasted material.
