Looks like a clamp for a old 2 man saw. It holds the blade when setting and sharping the teeth.
Here is a smaller version, http://www.ebay.com/itm/Set-2-Clamp-Vise-saw-sharpening-tools-Sargent-Co-90-1-unmarked-/170988913863?pt=LH_DefaultDomain_0&hash=item27cfbbc4c7