It's a First Style Reinforcing Wedge. The earliest example found to date is an 1893 A1 with damascus barrels. It is possible that the wedge was added at a later date.
Scroll about 1/2 way down here for other examples, and I added your image
http://www.picturetrail.com/sfx/album/view/15234180
David Williamson on the LCSCA site has a better DOM list for Syracuse and early Fulton guns.
BTW: the barrels are Laminated Steel and the rough forged tubes were from Birmingham; note the Provisional Proof mark.
Threaded Mode