PSUengineer1:
It looks like a special sized brick. It appears to have about the right proportions, height to width, but it is longer than normal, about (21" minus a head joint)/2. It looks like it might have been made for chimney building and to allow the use of a 12" clay tile liner. Is there a clay tile liner? What are the actual dimensions of the bricks and what are they made of? Your photo really doesn’t show that very well, and the mortar joints tend to blend right in with the brick material.