Old and 'new' castings. Note the amount of lost material around the spindle boss in the old case. The old case also exhibits a high degree of corrosion on the inner back plate in addition to some shaving by the back of the impeller.
The small hole visible on the right is to a small vent connecting the inlet pipe (on the other half) to the outlet pipe. This presumably relieves any pressure differential created by the impeller. If anybody can add to this please let me know. It does not appear on the original General Arrangement drawings.
The rear castings that form the inlet to the pump. Significant corrosion on the old case with a large amount of material planed off the inner face by the impeller. The thrust adjustment screw in the centre of the old pump had almost worn away.