That is a second bracket for the sight that is on the tang. It is used at VERY long ranges. You recline on your back with the rifle's barrel resting on your foot and the butt in your armpit. It looks very uncomfortable to me, but is a very stable aiming system! It was very popular when that rifle was new, and fell out of favor when Single Shots rifles fell out of favor.
The BPRC people use cross sticks for the same purpose (to elevate a very heavy muzzel to get sufficient elevation on long shots.) The Irish National Team were the masters for many years when shooting 800 thru 1200 yards (880yards =1/2 Mile.) They may or may not have created that system but they gave the rest of the shooting world fits with it! I believe no external supports were allowed in the old days. No shooting sticks, bipods, tripods, or bed rolls, & ect!
A related firing stance evolved from it in the late 1960's with the SS rifle caliber pistol hunters taking long shots on game, from Prairie Dogs to ELK. In the modern stance you support your neck or upper back with your non shooting hand, I cannot stand the contortions I have to do in order to use it, BUT it, like the system for the old rifles, is very effective!
In the 1880's the Creedmoor range on Long Island, NY was THE hot bed of long range competition in the US. Remington Named a model of their Rolling Block Rifle after that range, like some companied do today with Camp Perry. And of course, Hornady uses that name for their long range cartridge family (22, 6mm, & 6.5mm Creedmoor) After that range closed down Camp Perry became the hot bed.
Ivan
BTW, If you look at the front sight, this is where you adjust for windage. On this model, the eyepiece of the rear sight is loosened and manually slid up or down for elevation (unlike Quigly's far more expensive rear sight) Modern production of all the popular sights are made by Montana Vally Arms. Their online and paper catalogs give prices and usage instructions. I have a Sharps Vernier Long Range model on my 1874 Sharps which has about 15 MOA of windage adjustment also.
