I have been through all my German sources, without success, and the internet offers the usual everyone copying everyone else without checking (or mentioning) sources.
There seems to have been a couple of Americans flying as observers with VB108 around this time (Norman Prince won a Croix de Guerre ?), so there may be some information from US sources, though those I have seem to start the story with Lafayette just as the German sources (and others) ignore aerial combat prior to the Fokker.