Found this laying inside rear door, is it a window guide broken off?

Number on it is 16609326 RH-2

It looks like it, but the window goes up and down ok.

It is a strong fiberglass reinforced molded piece.
I would like to reattach it if I can.

pics here

https://photos.app.goo.gl/5Vi6gjvDMh6LNgk46

https://photos.app.goo.gl/B7cMHL2uA4u3zmHe6

might be part of the guide rails on the sides that the window goes in?

^^^^^

That's what it is. Since your window goes up and down just fine, it's nothing to worry about.

It's broken and cannot be put back on anyways.

Ok, I did verify that it is an extended support holding the rubber window guide.
And not sure why it was needed as the window goes up and down ok, the window does not go very low anyway.

I will look at it more closely. maybe it helps keep the window in a certain angle as it goes down. Just guessing.
Likely I will lay it back in the bottom of the door.

I don't want to remove the glass just to try and epoxy it back in if it is ok.