Nothing valuable, but I am curious as to what this is?

Any ideas on this two-piece brass "window frame"? Has brass rivets holding both halves about 1/2" to 3/4" apart if they were spread. Rivets in each corner, but only rivets halfway between them on two sides. Found in an area with all sorts of targets from the last 100 years, so, no hint as to age other than how it's built. Age and use would be what I'd like to figure out. Pilot light window maybe?

 

Be there remnants of Mica between those frames?

 

It looks dead bang, size-wise, for a 35mm slide. Some of the early ones had metal holders. If that was dug, the slide and the paint could be gone.

 

Uh-huh...but then why the 1/2" to 3/4" thickness?
EDIT: I think more likely it's a frame for a pyro-ceramic or glass window into some sort of firebox...bicbw

 

True. Glass slide of some kind maybe?

 

@Rob Langdon , can you tell what the pointed remnants showing at the top of the opening are made from?