The first unit, 316999, was the AC conversion of the Class 457 prototype DC unit, 457001, which was itself a conversion from one of the prototype second generation Diesel Electric Multiple Units of Class 210.
The first unit was the AC conversion of the Class 457 prototype DC unit. The Class 457 prototype DC unit was 457001. The first unit was 316999. The Class 457 prototype DC unit was itself a conversion from one of the prototype second generation Diesel Electric Multiple Units of Class 210. The Class 457 prototype DC unit was 457001.