I had that Fairey overdrive in my '67 Series IIA. It really was sweet. the best part was that it basically split all the gears so you could cruise in 3rd "and a half" when it couldn't pull 4th and 3rd was an over rev type of gear.. It bolted on in place of the PTO cover and replaced the tranny output gear which made it easy to install.
I did understand that they weren't the strongest though... but I tourtured mine (as bad as you can with a 77hp motor) and it did OK.
made a cool noise when it was engaged too, and the extra lever was always fun. I wanna say I got mine from www.roversnorth.com
but I could be mistaken.. it dosen't look like they sell them anymore.