The key would have been the interior pictures. The size of the locker on the LHS tends to give things away.
the next key is the width across the tub. 4' is a 4 4' 10" is a 41/2 5'10" is a five. These later gained the names Dart, Delta and the Discovery. The designer is the same size as a Discovery but is configured differently.
The lock may have the key number on the tang at the back of the tumbler.
The brown Dandys were a time of development. The Dandys had adopted the plastic sides that didn't last for long before steel was adopted. The one you have has flaps on the front wall to attach an awning. Awnings became full width later on. Then the final change was the adoption of the sandwich colour schemes with white or coffee sections in the wall.
Still waiting for the details behind the pen name and thought 1941 was a bigger year.