We know the MGA 1500 has all drum brakes, and the 1600 has front disc brakes. Also the 1500 has a flat top cover on the master cylinder, while the 1600 has a tall cover to contain more fluid volume required for disc brake calipers. But now Gary Lock from Gold Coast, Australia came up with this discovery.

The cover on right is for the MGA 1600 cars. The much taller cover on left has unknown application, but most likely would not fit under the body cowling of an MGA. Just another odd part to watch out for if you ever see one.