Very early buses had large rear hatches for access to the motor; these early buses are called 'Barndoors'.
The crank is for starting the motor.

Below: A barndoor spotted on the Autobahn in Germany.
Farther below: a pair of barndoors, British Volkswagen Festival 1997.

Below: gallery of other vehicles.
convoy scoop Barndoor radar bus Samba
Halftrack Draisine Kombi panel van SnoMo
ambulance floridaboy freak van fire! doublecab
camper Pullizei
speed T2