This just popped up on eBay.  Seller doesn't know what it is and neither do I.  Anybody?

http://www.ebay.com/itm/SMALL-OLD-CAST-IRON-CARPENTER-FINGER-REST-TOOL-OR-PART-NOT-REALLY-SURE-WHAT-/261322517494?pt=LH_DefaultDomain_0&hash=item3cd8092bf6

Leather slitter.

That type is called a lace cutter, used to cut laces and thin strips of leather.  They usually go for around 20.00.

Thanks for the ID.  It's now on my watch list.

Note the blade made out of a piece of old hacksaw blade with the teeth ground off....

Larger versions, known as ploughs, were used to cut straps and belts from thicker leather...