First, a caution-note. The following info is JUST a "rule-of-thumb"... which means, there are some exceptions to the rule. It's not an "always" rule.
Usually, when a BRASS spur's strapholder has two rectangular slots and the strapholder has a sloping connection to the spur's "arms" (like we see on yours), the spur is Confederate-made. Yankee spurs tend to have a 90-degree junction where the strapholder joins the "arm." To see what I'm talking about, compare that area on the brass spurs in the photos below. Remember, I'm only talking about spurs with two rectangular slots in the strapholder.
I know there is a collectors reference-book on civil war spurs, but I don't own it. Perhaps somebody here knows its title, and can check it to find an exact match for your spur.