Looking for Info on a Sheffield Folding Knife

[44-henry]

I found this interesting knife this afternoon at an antique shop near Lancaster, Pennsylvania. The knife is 5" long overall with a blade that is just a hair over 4". It has an interesting lock feature that extends out the back. It has very thin brass liners (about .020", bolsters, and black horn scales with steel pins. The blade has a high polish with a fine finish and has not been messed with, though it has some staining and light corrosion. The horn handle is broken off at the tip on one side. The blade is deeply stamped with "Superior Steel" and the Sheffield mark is barely visible along with some other marks that I cannot make out. It is obviously a very fine quality little folder and appears very old, though I am unsure about the period that it might have come out of. I doubt it is any later than 19th century, but I am not familiar with this style knife. Any ideas or info would be appreciated. Sorry about the quality of the photos, I was using my cell phone and haven't had a chance to do anything better yet.