The Third Reich Heer(army) and Luftwaffe(airforce) rank of Unterfeldwebel is equivalent to that of Staff Sergeant in the current day US Army and US Airforce. As far insignia went, an Unterfeldwebel would normally wear two stripes on the arm and a single green bar at the collar. An Unterfeldwebel of the Fallschirmjäger would wear 2 eagles .
The Kriegsmarine equivalent of Unterfeldwebel was called Obermaat whereas the Waffen-SS equivalent was the SS-Oberscharführer.
For a more complete overview
of which ranks existed
, see Wehrmacht ranks
The page that helped me with this writeup is www.wehrmacht-association.com/Open_Pages/ranks.htm