Newton's Rings?

I've been cleaning out my accumulated ATM stuff - the stuff I've gathered over the years with the thought "This might be handy someday." I came across an interesting mystery. It is a 4.25" ~f/4 uncoated mirror about 3/4" thick. When held just right in diffused sunlight (what a photography might call "cloudy bright") it nicely shows colored rings. I've attached a photo.

I'm posting this here because I suspect they are Newton's rings, which I could see in my 5" f/15 refractor objective using a fluorescent light. The rings I see with this mirror are much broader. They do not shift position when I change the tilt of the mirror slightly. The look like they are right on the surface. I can see them through the back faintly, and they still appear as if they are on the front surface.

The back of the blank is flat and very finely ground. My guess is that the blank is free of wedge, and that it is an interference effect between the front and back. An alternative might be some special coating on the front.

The white circle is a sticker on the back of the blank.

Anyone have suggestions or thought on this?

Thanks, and clear skies, Alan

Attached Image:

I'm fairly certain the pattern is due to stuff deposited on the front surface. It reminds me of the Edmund Scientific "Astroscan" scopes. These were 4.25" F/4.2 and the mirrors were (according to rumor) parabolized in the coating chamber by depositing stuff of varying thickness. Perhaps thats what you have here, sans the Aluminum top coat.

http://www.scientificsonline.com/astroscan-plus-telescope.html