When we had our first Sed Ak 400 at Tilcon, a P Reg one with 6LXB Gardner, it came fitted with the David Brown six speed gearbox but within a short space of time it went back (presumably to the factory?) and returned with the DB eight speed range change box fitted. Why this was done I have no idea, or who paid for it? Presumably it was a factory recall but I never did know the reason behind it, I will have to ask Maurice Fearn who was the driver at the time as he should know. Then what happened to the original six speed box(es), were they then resold? 
Did this sort of thing happen a lot regarding operators wanting a change of engine/gearbox etc after they had taken delivery?
Pete.