It is a heavy duty version of the 904 - 5 disc front clutch and 4 disc rear as opposed to the 3 disc drum for a standard 904. Stick a low gear set in it from a later unit and you get a bullet proof trans capable of handling a lot of abuse. Some say its worth a tenth over a 727 in a drag race application. Less rotating mass or some such thing.
Its also difficult to try and build one from a regular 904 because the heavy duty parts do not interchange. Best place to start is with an original unit but they are hard to find. Basically only came beind 360 powered pass cars from 74-77 before they went to a lock up style. (Lockup not being desirable for my application).