It is "in the books" as a civil war yankee Officer spur. Specifically, a "Spur, Model-1859, Federal Officer, with scroll adornment."
http://relicman.com/miscellaneous/zArchiveCavalrySpurs.htm
Note, officers from every service branch of the army tended to ride a horse instead of marching, so your spur may not have been worn by a Cavalry officer. Having a "home-made" replacement rowel indicates (but isn't absolutely conclusive proof) that at the time of its loss its owner was a Confederate cavalryman. A yankee officer is far less likely to have gone to the effort of crafting a replacement rowel from on-hand material... and would instead either requisition another spur from the Quartermaster or purchase one from a sutler.