Ok, through my cleanout of the outside of the chimney I stuck ruler to try to gauge the size of my chimney. (trying to determine what size brush to buy) Its rectangular...... and from the inside of the masonry one side is 10" the short side appears to be 4" Does this sound right? I see alot of 10x6 brushes on the net.... No 10x4. Could this sizing be OD (outside diameter) not ID? Any help appreciated.