Simon
Main engine exhaust manifold.
The crossection drawing shows engine looking towards the forward end , see drawing below, the reversing cylinder up front is removed on the later engines. There are no indications of any objects or restriction flanges towards aft and I can not see any reason why such restricton in exhaust manifold diameter should be justified.
If you look at the photo below showing the forward exhaust manifold endcover there are two reasons why this is not the main endcover, only 6 small fixing bolts whereas on the exhaust manifold flanges you have 12 fairly sizeable bolts, there is obviously a large hole in the cover for the reliefvalve. Based on this my assumption would be the shown forward endcover is just a protection cover and behind is the cast double wall cooled end dome being the real cover. In the end dome is an inspectioncover having a smaller diameter than the exhaust manifold and the drawing shows this inspection cover.
Tore