I did some research, and apparently, “United States” without “of America” could be a kind of ellipsis. But more likely, it’s just an alternative country name. So I think that makes US an initialism (because you pronounce it as [yu-es]) for an alt (bonus info: this is a final clipping, or apocope, of “alternative”) name.
Linguistics is such a dirt hut…
You know, it just kinda clicked for me, a lot of people can relate much more to the bottom person than the top. There’s quite a few people who scoff at education. Call people eggheads and stuff.