Thank you for providing the measurements and the back-photo.
At the time of the civil war, and a few years after, the US Cavalryman's insignia was made of thin "stamped" sheet-brass. Your insignia is thicker, being cast-brass. That dates it from the mid-to-late Indian Wars era at the earliest, and could go as late as the present. To narrow down that very wide date-range, we rely on the insignia's "form of attachment" and details of the emblem's design.
Your US Cavalry crossed-sabers insignia's back shows small spots of solder on the ends of sabers' scabbards. The solder was used to attach a pin-&-clasp fastener. See the photo at the end of this post.
The back-photo shows tiny
brazing spots on the back of the two scabbard-rings near the junction of the sabers. Those brazing spots held the upper corners of a wide letter (such as an H), which is now broken off. Again, see the photo.
About the details of the emblem's design. Note that on your insignia, the saber's handle (called the "grip") shows parallel ridges. The saber's guard shows two thin branches. Also, there is no "saber knot" on the lower part of the saber's guard. The sabers' scabbards are narrow/thin in comparison to other variations. Carefully noting such design-characteristics, I believe the photo shows the exact insignia-design and attachment-method, listed as being a US Cavalry Officer collar-insignia, from the "early 20th-Century." Scroll about halfway down the page at the following website:
http://hglanham.tripod.com/uscavalry/cavalryinsignia.html