I just picked one of these up at a garage sale for $2 not knowing what it was. (and got a lot of other great buys too!)
It says C.S. OSBORNE & co. NEWARK, N.J. From what I’ve read it is indeed a leather cutter, aka draw gauge (http://www.csosborne.com/no51.5.htm).
Mine has black japaning on the handle and the blade says C.S. OSBORNE & CO HARRISON N.J. The blade must have been a replacement.
On the opposite side of the measurements is it stamped MADE IN U.S.A., it’s stamped again U.S.A. in smaller letters, and then someone felt obligated to etch U.S.A. in it again…
It also measures out to 5”, most listed online only go to 4”.
What I understand is that the company moved from Newark to Harrison in 1906 and the first patent that they had on these tools was 1876 I believe. So it is at least 106 years old today. Most likely older. They seem to be averaging about $50 online regardless of the condition (obviously a bit more for better shape).
I also am not exactly sure how this was used because as you can see in the link that I posted it as appears as my blade is in the wrong way.
Anyways, I am going to keep it. I actually was using it as a type of marking gauge until I looked into what it supposed to be used for. I still might use it as a marking gauge, need to sharpen the blade up some. It works best as a marking gauge if you flip the bar upside down and keep the blade pointed down, then you have a square edge to ride along the edge of the wood. (As shown below)
Also, I believe mine is missing the gibs that go between the bar and blade and the thumb screws (still works)
Great find anyways, cool conversation piece!
-- Grant - N Dakota