good guess again but not correct here's another clue.
I'm looking for an Airplane:
1. Twin engine.
2. Two distinct variants were built and other variants were modifications to the original airframe after it was retired from it's primary role.
3. It used turbo-props for thrust.
4. The standard version had a crew of three.
5. It served with four nations including the nation of origin.
6. Changing the design from the prototype to include the third crew member resulted in a disturbed air flow over the horizontal stabilizer which was counteracted by fitting small 'finlets' on either side of the vertical stabilizer.