People keep asking about this variation of intent documented in the Workshop Manuals. For MGA the book says to position the piston notch downward in the caliper. For MGB the book says to position the piston notch inward in the caliper, facing the bearing spindle.
At left, MGA caliper with piston notch downward. At right, MGB caliper with piston notch inward.. By the book.

My best wild guess is that some smart ass engineer at the Lockheed factory thought it might have something to do with even or uneven pad wear, but changed their mind later when more field experience might have been available. It is possible that positioning the notch in the direction of rotor rotation (downward) may have resulted in detrimental effects on pad wear, and re-orienting the notch perpendicular to rotor rotation (inward) may have cancelled out that effect (same as having no notch). Personally I don't think it makes much difference, and the pistons could have been made without the notch. But if we are to pay any attention to it, I'd say follow the later instructions for the MGB.