This stove was a FB Marketplace find locally for $50. I knew the guy and was able to get to it before anyone else. The top of the stove looks like it maybe had a collar of some kind for a connection to a liner. The metal is rough like it was cut off. I believe this to be a common insert so I thought someone may shed some light on my curiosity. It looks to be in good shape with no cracks in the secondary air system that I can see at this point. I have painted it and replaced gaskets. Hope to get it installed soon and begin the painful wait for cold weather in central NC. Been burning wood for a few years and have gained quite an education from this site.