The Hurricane's rear fuselage has a "D" shaped cross section with squareish lower corners. The whole lower part of the fuselage could be unbolted for access. There is a good photo of this at
this site showing the Finnish Hurricane's restoration.
The high contrast between the white and the exhaust stained area is partly due to the white paint being stripped off there by the hot exhaust. The exhaust staining itself is fairly weak from the cockpit back.
The underside of Hurricanes generally received a fair bit of oil staining from the engine as Merlins were fairly leaky engines. Also there was the mud thrown up by the propeller and tyres. I'll consider increasing this staining a bit before I submit it.