ahhhh yes to be 38 again....... I have one of these, not the same manufacturer tho..
Its used to set the kerf when sharpening your own hand saws. Mine is adjustable for cross-cut or rip or fine trim saws. Adjusts for thickess of saw blade and the degree of kerf / toot set you'd like. I remember my grandfather sharpening his own saws.. wish I could have gotten the "wide" vise he used, but some other SOB pilfered it as I was too young, but I fooled them (me thinks Mom pilfered it and I found it later with other goodies), I think the unit I have was his, although missing the "vise". I have the two man saw and remember him taking many hours sharpening it, don't think there was a kerf on the two man saw tho.