Cross-section of the MGB brake caliper from the Leyland Workshop Manual
Animation showing how the piston seals in the caliper are distorted when fluid pressure is applied and the pistons move out, which pulls the pistons back in as the pressure is released. (Motor Vehicle Maintenance & Repair Stack Exchange)
Caliper piston showing the cut-out (Brown & Gammons)
WSM drawing showing the cut-out facing the centre
MGB pad with piston witness marks ...
... showing that without the cut-out the lower edge of the piston would be off the pad ...
... but with the cut-out it is effectively in the middle and will apply even pressure of pad to disc.
However, when that is combined with the shims on V8 pads which have a gap across half the piston, who knows what the effect might be?