All units were formed of four cars. When originally built, units were numbered in the range 01s-32s and were composed of two outer driving trailers, an intermediate trailer composite (i.e. with some first-class seating) and an intermediate motor brake. The units were constructed to operate off the 1,500 V direct current (DC) overhead power system used on Eastern Region suburban lines from Liverpool Street to Shenfield and Southend Victoria. However, in the late 1950s / early 1960s, these lines were converted to the 6.25 kV/25 kV alternating current (AC) overhead system, which was adopted as standard and coincided with the introduction of new Class 302 (AM2) units. Therefore, from 1960 to 1962, the entire AM7 fleet was extensively rebuilt at Eastleigh Works to allow units to operate from the new voltage system. The work including moving the guard's compartment from the motor coach to one of the driving trailers. At the same time, units were renumbered into the range 101–132.