194 standard four-car units were built between 1967 and 1974, numbered 7701-7894. They were subsequently renumbered in the 1980s into the 30xx and 31xx series, in order to make numbers TOPS compliant. Between 1988 and 1990 the units were internally facelifted. The work involved fitting fluorescent lighting and public address equipment, and also rebuilding the MBS vehicles with an additional two bays of seating in space formerly used for luggage. On refurbishment only the first twenty units were renumbered in order, in the 34xx series. All other units were renumbered from 3421 onwards, in the order they were refurbished. All units ended up swapping motor vehicles on refurbishment - the additional work on these cars made the work take longer than that carried out on trailer vehicles.