Mark FV8128 is not actually the design designation of the trailer, it is just for the early cargo body that sits on a FV8127 Mk 1 light sprung 1 Ton trailer chassis, which together form Trailer, 1 Ton, 2 Wheeled, Cargo, Mk 1
Later versions of FV8127 were fitted with FV7970 cargo body that was fitted to both light & heavy sprung trailers. At that stage these trailers were recognised as being in the FV2401 series of 1 Ton Trailers.
FV2401 sequence fits into FVRDE classification of designs for vehicles & trailers of their various kinds. The FV800 series was allocated to 8/10 Ton, 3/4 tracked vehicles. There was no FV8*** allocation in this sequence.