Hans-Joachim Marseille jacket

[Rory Schultz]

..looks like a motorcycle jacket too me. I would guess that this type of Jacket came in use on racing motorcycles and aviation as well, although they might have been intended for different purposes. Shoulder flaps could have been added for military use. It was not uncommon to utilise clothing for different applications than initially or originally intended for.
Does my gibberish make sense?

The shoulder strap ( on motorcycle jackets ) actually started with the Motorcycle police units from the 1920''s and 1930's it was used for the crossover belt that attached to the waist belt that carried their weapon and ammo pouch just like the Military had. During WW1 up until WW2 Aviators were still using long flight coats and this was changing during the mid 30's because of the use of parachutes required the coats to be shorter. You can not wear a parachute with a long closed heavy outer coat. It was just a matter of convenience leading to the use of short motorcycle jackets, until aviation companies started contracting them for pilots. Can you imagine trying to get out of a burning plane wearing all that gear? That is how a ton of B-17 crews died trying to get a parachute back on during a flak and enemy attack.