DSC,

The index slot while not increasing strength much WILL insure that the plates are ate 90° and serve as an index point for mounting removing most if not all possibility of twist (or slant) of the plate being bolted to the main plate. While this does add a bit of machining the benefit both in initial set up as well as any maintenance down the road can NOT be under valued. The little extra added effort will pay you back a huge amount further down the line.

Michael