In Pirkles book, it does state that the nose cap on some rifles were drilled for swivels that were never fitted. This is partially true for some serial ranges in the model 1866, but NEVER in the model 1873 or 1876. The nose caps on these guns were never pre drilled, unless the gun had swivels attached. Except for earlier serial ranges in the model 1866, sling swivels were always a special order extra on all Winchester sporting rifles and carbines, but not often listed as such in a factory letter. I have two nice 1876 Winchesters with sling swivels. Both guns are in exceptional condition, one being a standard octagon barreled sporting rifle and the other a very fine deluxe. Neither one of these guns letter "sling and swivels", but both guns were fitted at the factory and shipped as such. Once you start looking at factory installation, it is pretty easy to tell if swivels are original to the gun or "add ons". Only the Henry and the 1866 had the same style swivel, while most of the later models had a completely different base and loop. Check out the two photos posted above by 1988 RRC. The nose cap has a flat spot milled on the bottom, just big enough to fit the base of the loop mount. The shaft on the mount is put through a hole drilled in the center of the flat and then a steel washer is peened to the shaft on the inside of the nose cap. If you are familiar with a copper saddlers rivet and burr, it worked the same way. It is peened just tight enough to allow the swivel to turn without being loose. The rear loop and base are almost ALWAYS 4 1/4" from the outside of the butt plate toe to the center of the base. I have seen very few that don't measure up to this distance and most that don't were not installed at the factory. Screw in sling eyes are very rare on large frame Winchesters like the 1876 and 1886. These models were simply too heavy to support the eyes. The eyes were used extensively on model 1892, 1894 and 1895 model rifles and carbines. Hope this is of interest.
Adios,
Buck