This black leather frog is pretty typical of those found on Winchester Model 1866 and Model 1873 bayonets.  The pocket is stitched to the body and then riveted on each of the four corners.  The pocket is slit from the top edge to the lug hole by design. Probably to facilitate adding and removing the scabbard.  This specific example was found on a very rare Winchester Model 1866 bayonet.