Complex sentence: Of these two letters the first had to be shown in upper case, the second always in lower case, despite its origin--thus, "Fw" for Focke-Wulf or "Bf" for Bayerische Flugzeugwerke.
Split into simpler sentences: Of these two letters the first had to be written large, the second always small, despite its origine. Therefore "Fw" for Focke-Wulf or "Bf" for Bayerische Flugzeugwerke.