Heat Riser In Exhaust

[RalphG]

I wonder how many still have the heat riser flapper valve in the exhaust manifold pipe like my Mercs do? Located at the front of the right hand exhaust manifold. I don't know for sure if mine works but it does turn by hand and the spring is still there. I don't recall if the 49 Merc truck has one or not. Maybe it is just a cold country option? I took this poor photo of mine today. It was below zero in the workshop so I didn't want to have my fingers frozen taking pictures.