Might not be Civil War, though, and from here looks more like a plate than a buckle. Also, it really could be a decorative piece. That's why more details are needed. Likely from this hemisphere and likely military, but...
It is of a design and style known to have existed in the pre-war era, however also remained a generic popular design for many decades after the war. It would take close examination and comparison to determine which it might be.
I've seen one before that was dug when excavated the Gettysburg battle site I understand they put it in the Jennie wade house not sure if it's there now. It was gilted with silver. I dug this in what's now a ghost town that is now on a long abandoned road nearly consumed by the forest that once stretched from kingston all the way through margeretville New York and beyond. I've been told by a handful of old timers that many civil war vets once lived near where I found it as it was remote and they enjoyed solitude. It was further from Kingston than margeretville. It matches the pattern from the cs wreath buckles used gone with the wind but obviously lyre instead of cs. There is a bit of gilt left on the back it as it mostly melted off because I dug it out of a burned foundation one that burned long before 1939 so no Hollywood fake.It measures three inches long and two and one quarter inches tall and is the right thickness for a single piece stamped brass buckle which it is. I've gone to great length to find its origin and in my opinion it's one of two known" as far as I know"
Your right about the pre war bit and it going through and beyond I've got that much the ones I've seen with brass lyres and wreaths that are post are completely rectangular
Interesting find, take a look at Antique- Indian War period, US Military Band brass belt plate/buckle 1870-1890 listed on e-bay, has some similarities to your find.
I don't think this is a wartime type of attachment - the wide tongues are made to attach to a simple wire loop on the opposite end of the belt. Wartime examples tend to have narrower tongues that engage a keeper attached to the opposite end that has a smaller opening, like in this example of a regulation officer's eagle buckle:
Or this example of an enlisted man's eagle buckle: