A fighter (or Jagd) Geschwader was comprised usually of 3 Gruppen written I./ II./ or III./JG51. Each individual Gruppe was comprised of 3 Staffeln, written 1./ 2./ or 3./ Staffel in ascending numerical order. So 7./JG 51 is the 7th Staffel of that Geschwader and was in III. Gruppe ..
Staffel numbers were colour coded of course for ID purposes ....a fourth Staffel per Gruppe was added from mid-late 1944 along with a fourth Gruppe to each Geschwader. The number '13' (or '1') would usually be the a/c flown by the Staffelkapitän while the Gruppenkommandeur tended to use the double chevron as an ID marking. The CO of a Geschwader was a Kommodore. The Gruppe can be identified by the symbol (or lack of it ) aft of the fuselage cross..
A note on the written form of these words. The plural form of Geschwader is ..Geschwader - there is no 's' or 'en' or anything else in the plural form, in fact the letter 's' is only very occasionally a plural in German. Most usual plural forms being 'e' or 'en' or just 'n' among others. And as we've just seen some German nouns have NO plural form.
I'm sure you all know that one. Now go and look at the text in your average Osprey title - even though Weal speaks German I reckon his correctors and editors don't, but writing staffels, geschwaders etc etc is incorrect in my view. Feel free to cringe when you see other English writers who should know better coming up with 'Geschwaderen' or 'Geschwaders' or similar 'non-grammatical' forms.
All nouns are written with a capital letter in German, eg the German for 'ace' is 'Experte' which becomes 'Experten' in the plural.
Abschuss is one kill - " his 150th Abschuss "
Abschüsse is several kills - " he achieved 150 Abschüsse "
Genitive or dative case plural forms ('n' or 'en') do occasionally throw up some 'curiosities' in German. For example;
Mit 352 bestätigten Abschüssen ist er der erfolgreichste Jagdflieger - "Mit " (with) always takes the dative case in German and the dative plural ending is '..n' so in this sentence " with 352 confirmed victories.." you would write Abschüssen in German but since cases do not exist in English I personally see no reason to write these German words 'incorrectly' in English (if you see what I mean..)