question about "tube" inside wood stove jacket

Hi,i recently picked up an old Stratford Foundry No.14 woodstove. The fire box and ash doors in the front. It is cast iron in a metal jacket. Now for the question....In the rear of the stove.inside the jacket,is a square "tube" that goes from the top rear to about 1/2 way down. What is this for?.I,ve seen this before and I believe that it had something to do with a thermostatic damper,but this sove does not have one....Larry