That is not a standard cover, but the "door" looks looks a factory made item. It unhooks from the left and pivots to the right. Is it Ford or borrowed from a later vehicle?
Underneath the cover is the top of the Carburettor/Air Cleaner. The Door doesn't look large enough to remove the lid of the Air Cleaner, maybe to prime the carb with a squirt of fuel on a cold morning, so I don't see what purpose it could be for
Do you know if the engine that was beneath this cover was modified in any way?