YUou're exactly correct. I have the same ashtray. I was informed from my inquiries of your same thoughts. Its from 1976 and is the bicentennial ashtray.

In all of my ashtray searching I have only seen the one (which I bought). So I'm assuming its on the rare side. That's just an assumption, I could very well be wrong. Perhaps the readers on the board could give you input on if they've seen them around.

But on the other hand, whether it's rare or not, its still a great ashtray. Very cool and a nice find.