From what I was told by Marv long ago, these should have been called
BONE-A-DRIVE, as you get boned when you buy one. There are no service parts for these units, and were not really that good from the get go.
I have one on the back of a three-speed case, but it was not ever run in anything that I have owned...
I have just removed an overdrive unit from my FJ40 (have a 4 speed trans). It is not the one you're speaking of, and not a Fairey, but I had it on my truck for 2 years without any problems (except for a little oil leaking - most likely my fault sealing). My daily driver is being turned into a trail rig, so I don't have a need for it.
I had to do a little shimming to get it to work, and I got replacement bearings from a local supplier (used a Torrington bearing). It is a 25% gear reduction. Not too difficult to install if you are familliar with removing the transfer case.
It's a hone-o-drive. It's a primitive and failure prone POS. It's not worth the effort of installation much less the price. Because it's round, it doesn't even make a decent door stop. It's a pain to remove if you want to salvage the transfer case, which is the only worthwhile componant there. Even then you would have to replace the lower transfer shaft which has been modified.