The whole thing is solid and looks to be brass. two places on the shaft are threaded. Those nails are not to bad for someone who likes to play in the dirt all day.
Any civil war weapons experts out there? What are the chances that it's the knurled screw adjustment knob for making sight adjustments on a rifle... or... any other adjustablle equipment componant from that time? I was thinking medical equipment (like Michelle pointed out), weapon, optics... whatever...
Seems the two threaded areas on the neck adjusted soemthing when turned.
But... if it ends up that daddytopcat is correct, then I agree with him! 8)
Could be an adjuster from a carpenter's combination square or a knob from an engineer's sextant. Lots of things had brass adjusting knobs between then and now...