Yes, this is a very confusing subject but we (Larry deZeng and I) think we have it sorted out. The most critical information comes from original Luftwaffe documents but there are other clues too.The comment about St.G. 76 having the “F1” code was common knowledge for decades but when I checked into it I found no actual evidence to support this anywhere!
According to our next book (to be titled something like “Ground Attack Units of the Luftwaffe, a Reference Source”) which should be released next year, we have concluded the following:
I./St.G. 76 transferred to Barly (17 km west-south-west of Arras) during 25-30 June 1940. The remains of Gruppe disbanded, with the greater part going into the creation of III./St.G. 77 and a smaller part going into the formation of I./St.G. 3 on 9 July 1940. The Kommandeur of the Gruppe during this time was Walter Sigel. (i.e. for BOTH Gruppen).
We postulate (in the absence of real evidence) that the unit codes for this/these Gruppen was actually
S1+_B, H, K, L). Larry Hickey says he has evidence to support this but I haven’t seen any myself, so I would be most curious to see it.
As for III./St.G. 77:
We have that it was formed on 9 July 1940 at either Barly (17 km west-south-west of Arras) or at Caen /Normandy from the major part of the former I./St.G. 76 with a smaller part from the former II./K.G. 76.
Codes: up to July 1942 (F1+_C, M, N, P), then (S2+_D, R, S, T).
I hope this helps to clear things up.