amazing azza,
Before you close the book on this question lets consider another possibility.
Pump Manufactures do not only make castings for one commodity or service. Long ago I was involved in a project where pumps for the same service (mated A&B) were not the same casting. They were a slightly different size and looked different, but the Pump Curves were the same.
My point regarding your Pump question is: Your Pump case may have originally been cast for some totally different service. That other service required something special, maybe a Slip Stream Recycle, to prevent cavitation, who knows. The Pump Manufacturer had a left over casting that met your needs so they did not need to pour a new casting. If this little connection does not affect your pumping requirements then don't worry about it.
Sometimes its possible to do all the right things and still get bad results