If there is some component of Chopper's leg that would be needed to restore AP-5, surely that component could have been harvested from Chopper's old, mismatched leg. Alternatively, if AP-5 would be better served by the newer component, the new one could have been harvested and put in AP-5, while the comparable component could have been taken from the old leg and put into the new one, allowing Chopper to have a matched pair as he desired.