Removing 5" extension from Exhaust Tube

[Carl]

I have a 25" mercury 150 engine and a 20" mercury 150 HP parts engine. Same vintage so most parts are interchangeable. I am trying to remove the 5" extension from the 25" engine exhaust tube and it does not seem to move at all. I don't want to break or damage anything, - so what is the trick? I don't see any fasteners on the engine or in any parts sketches. Any help is appreciated. Thanks