I probably really don’t want to know the answer to this question, but, is that really the gasoline filler cap in the door???
Probably. The Army’s M151 Jeep had a filler cap there; because the driver sat on top of the gas tank. After awhile, we got used to it and didn’t pay it much mind. Not too much, anyway...
That is the filler cap for the front engine.
The left tank filler cap sticks through the left door, and that tank that feeds the rear engine.
The photo is of the Citroen 2 CV Sahara or 4x4 built from 1962 to 1967.