on second thought - couldn't Neftelberger possibly contain the common Jewish name Naftali ? With a German element added, which wasn't uncommon, AFAIK.
I've read somewhere that when surnames for Jews were made compulsory, in some areas one aim was to make them look and sound like German names.
But beware, this is guesswork only. Some of the Neftelbergers may already know the true origins and meaning of their name.