Quite simply, if you look at Barrett's contribution, "Victory Scores" at
http://216.169.126.3/contrib/vs.html you'll see that it was well known, at least now, that the French credited shared claims.
For the most part, the USAS used the rules of the sector they operated in, if they were in a sector that was under British Empire command, they followed those scoring rules, if the sector was under French command, that nation's rules were used.
As I understand it, once a sector was designated as being under US command, the USAS adopted rules similar to the French for victory claims.
VBR,
Al Lowe