Or, more acurately, the turbine from the turbo-supercharger. The turbine was located towards the aft, ventral portion of the fuselage, below and behind the intercooler doors, with the turbine exhaust just in front of the tail wheel doors.
The engine is certainly a twin-row radial engine. You can see a small part of the exhaust manifold still attached to a cylinder head in the center bottom part of the engine picture. I believe that is the accesory housing that is on the right side of the engine. Don't know definitively its an R-2800, but it appears to be so.