In the Brave New World of JAA-land, such a person would probably be a Type Rating Instructor rather than a Flight Instructor. Certainly so on multi-pilot aeroplanes.
Hence "Mr PBY" would be a PBY TRI; he wouldn't teach the 'big cows, small cows' stuff nor would he need to. But he'd be an acknowledged specialist on converting pilots onto the PBY. Lucky people! As to 'quality' of the instruction given - that's down to the individual TRI.
The rating qualification test would be carried out by a Type Rating Examiner (TRE).