I have seen a few long leaf ladder sights without the "1876" marking, but not very many. Most of the unmarked sights of this style have been on the model 1886. Even then, most of those sights are marked with that model date. I have encountered some long style buckhorn sights on various 1876 rifles, but I believe some of these were either replacements or possibly special orders. Winchester warehouse ledgers on the 1876 model are actually quite complete and most often will list any sight that was a special order extra on a particular gun. My hunch is that unless the factory letter says otherwise, your 225XX serial range gun was shipped with the standard long leaf ladder sight that has the "1876" marking. I also have a carbine in the 12XXX range, that has this very sight on it, but with markings on the reverse side. Because this is an early gun, the rear dovetail is too close to the receiver, and the sight is mounted backwards in the dovetail. The long leaf flips up forward instead of backwards and all the markings are on the inside of the leaf, not the outside. When the sight is in the up position, you can read all the calibrations and the "1876" date. Way cool. I also have several short leaf carbine ladder sights made this same way. Some are marked "1873" and some "1876".
Adios,
Buck