My 1873 doesn't have that screw, but my new 1876 carbine does. I didn't try drifting it without loosening the screw, which does pass through into the dovetail. It looks to me like, if the screw was tight, it would make drifting more difficult. I had to dig out a set of jeweler's screwdrivers to find one small enough. Fortunately it loosened easily--it would be hard to put much force on that tiny screw.
CC Griff