Showing a 3-main gasket on a 5-main pump, the large port in the pump is not covered and results in an air leak, low oil pressure and a high pressure rise time. (MGExp). The gasket on the right is the correct one with passages to match the pump: (Moss Europe)

The confusion comes from when the 5-main gasket is fitted to the 5-main block - there is no aperture for the additional hole in the gasket which leads people to think it is the wrong one: (1967 MGB GT)