I don't know much about them, other than what I have heard from a friend of mine who has a 21 Skater. His was rigged with twin from the factory, it can be seen in the OSO Classifieds under Skater.
They don't make them anymore.
I really don't know the exact years they were produced.
Not many were rigged with twins from the factory.
His name is LambRB on the board, you can direct more specific questions to him.