There are at least a few Glocks with early USA-made frames and Austria slide assemblies, and the frames are not marked "USA" or "Made in USA".
In late 2008, I ordered a G17C direct from Glock, Inc., which was delivered to me in early 2009 by my local dealer. The frame has a single line in the oval at the top of the right grip: "Glock, Inc., Smyrna, GA.". The slide is stamped Austria. Test-fire/born-on date was July 2008.
If the frame was made anywhere but in the USA and imported for retail sale here, it would have to have the importer markings AND "Made in (country)" stamp like all the other Glocks made in that era.
Photo of the frame markings: Top pistol, normal Gen3 G17 with two-line import marks. Bottom pistol, the Gen3 G17C that I described above, with a single-line mark and Austria-marked slide.
And a bonus frame marking photo, from a user who posted it here many years ago: