Which gearbox is it? Should be stamped/cast on the front face of the gearbox.
At some point FIAT seem to have taken the "real" gearbox sections out of the factory manuals and put them in a seperate volume which seems not to have escaped to the Interweb yet.
I have the gearbox manual for the C.501.5.10 in Dutch(!), but the suggestion seems to be that the box used on most FIRE engines, the C.514.5.10 can use the same manual, albeit that the individual parts are non interchangeable.