I always figured it was to assist in cooling.
That the exhaust would pull cool air through the fins around the head.
That's my theory anyways. Guess we will find out the truth soon
Alex

 

as far as i know, the tube has to do with the "forced-flow exhaust system" as stated in the manual for the 503 engine

 

Was helping to cool the muffler and avoid also condensation that leads to rust.

 

Hey i have one of these sleds as well 1982 ski-doo blizzard mx it has the exact same thing ski-doo refers to it as the "forced flow air system." Its for performance. The tube forces cold air into the end pipe of the exhaust. A hot engine with a warm pipe runs the best.