[Ls6 Ragtop]

Rich,
That's a cool plate. My research has yielded 4 colors of these early 1-USA plates. Green with White letters, Orange with White letters, Blue with White letters and Black with White letters. I have 2 black ones in my collection and whats interesting is the white lettering is a reflective sand type paint. The mounting holes are unique in that like yours, they are not quite oval in shape. They are heavy steel plates like the later issue White and Gold USA-1 plates. If anybody has any factory merchandising info regarding the issue dates of these 1-USA plates I'd really appreciate learning more about these.
Chris