I dont think you can ever date it, unfortunately...all you can safely say its an antique powder horn, and yes, the eye screw could have indeed been a later addition, however you need not definately assume a knob was once there, some times small nails were used to actually nail the stap to the plug, or used a simple wood screw etc...
You could perhaps be safest assuming that its from the second half of the 19th century? Sorry I cant say more but maybe others can.
tca