These heat exchangers are military surplus from military arctic vehicle heaters.  NSN is 2540-01-028-0568 PN is G738258.  These are new old stock in the box, the overall length of this piece is 11 and 1/2 inches, Diameter is 6 1/4 the diameter of the tube on the side is 1 and 1/4.  I believe the way these work is they convert vehicle exhaust into heat... I could be wrong.  Anyways they are brand new in the original box, this one was removed for display. Boxes may show some dust and wear from being stored.  Ships worldwide thanks for looking!