The Luftwaffe Stabsfeldwebel rank

I have a question regarding the Stabsfeldwebel rank in the Luftwaffe. How was it possible for someone to hold this rank? While it seems to be a rare rank to hold, I've done some research and found people holding this rank early in the war in 1941 as a bomber crewman, particularly a bordmechanik in a Junkers Ju-88. Wasn't the rank introduced in 1938 for NCO's reenlisting after 13 years of previous service in World War I or after 13 years of continuous service? From what I read, almost all the men who served in aircraft weren't even old enough to have served in World War I and the Luftwaffe itself wasn't even around for 13 years itself, so how was this possible? Perhaps I don't have a definite understanding of the rank. There seems to be confusion when I read about the Hauptfeldwebel rank vs Stabsfeldwebel and I've noticed sources seem to contradict each other when it comes to which rank was higher.

I've also attached a photo I've found on Flickr which shows what is claimed to be a Stabsfeldwebel with a canvas flight helmet. The same picture appears in the book Hitler's Eagles: The Luftwaffe 1933-45 by Chris McNab which claims the man is a stabsfeldwebel. How can one tell he is a Stabsfeldwebel if the collar patches are the only signs of rank shown, which are identical to those of an Oberfeldwebel?