Vtg Round Brass Mirror Bronces Artistico Espana.  This measures 15.5" but it measures 16.5 by the 2 attachment points.  On each side there is a tube that comes out and has threads in it.  So this is made to attach to a frame.  Please see the last 2 pictures.   This is what it looks like with the matching frame.   The glass does have a few spots I couldn't get to come clean.  It's hard to show in the pictures but I tried to capture it. The marking on the back is very interesting and I can't find much info on this piece.  Please see all pictures.