Note do not have a picture of the 1500 engined MG Midget rear brake backplates for comparison and whether these differ from 1098/1275 backplates (may be difference on location of wheel cylinder mounting pin hole?).
First picture below shows the rear of the rear backplates and the second picture below shows the front of the rear backplates.
Picture 3 above - comparison between early and later backplates and wheel cylinders - upper pair of backplates are early Sprite/Midget: note the sliding wheel cylinder is a Morris 1000 of 3/4 inch bore, rather than the 7/8 inch bore rear wheel cylinders for MkI & II Sprites that have front drum brakes (my 1961 AH Sprite MkII has front disc brake conversion). Lower pair of backplates are later Midget and Sprite with single twin piston wheel cylinder.