You are absolutely right Nellie. The Massin drawing used for the Fife tiara may have been sold to, and used by, another jeweler, who actually manufactured the tiara for the king of England. It seems Massin used to work for himself, for Mellerio and surely for other jewellers.
Another example comes to my mind : The Musy tiara made for queen Margherita of Italy is very similar to two other tiaras made by Boucheron a few years before.
The Mellerio Bourbon Parme Tiara is very mysterious in more than one way. Actually it seems there were a few of these tiaras made by Mellerio at the time. The motif used is called "lames" in the archives and it was rather popular at the time.
I can't comment more than this on the subject now, but I will able to do so in a few months.