Round fin/rudder type is the original as fitted by Dehavillands. The squarer type was fitted by Riley during the conversion process. Optional extra so to speak. Really only to "modernise" the look and so make it a bit more appealing to the yanks. Suspect not so much fuel contamination problems, but tired and U/S engines and systems, and very little money to repair or replace................Hey didja know them 720's were turbocharged as well when they left Riley's?