another forum some years ago which even provided suitable diagram of MK 16 fuel system to explain the reason
Think it may be understandable. The main fuel cock was below the instrument panel between the pilots knees, the rear tank cock was on the left and the drop tank cock on the right of the pilot. The manual notes (1949) that the rear tank use was prohibited and mishandling of the cocks may allow air to be drawn into the fuel lines. From the manual.
