Chuck Porter created this car from a wrecked 300-SL.  With its twin small-diameter exhaust pipes and "C-Modified" classification I would guess this photo shows the car when its original 300-SL motor was supercharged***.  The car later became a contender with first Buick, then Corvette power. 

Note Porter enjoying a stogie during the event.  I wonder how this would go over in today's safety-obsessed racing world?

*** Supercharging moved a modified car up a class in this era.  Hence a 2996cc Mercedes engine would be in class "D-Modified" since it was under 3000cc.  With the supercharger it would be bumped into class "C-Modified", competing for class honors against cars with engine displacements up to 5000cc.