From the mid 1920's, pairs of swing door motor cars were used to haul steam hauled carriages to the end of electrified sections. Steam engines were then used to haul the trains to their destinations. Return journeys were worked in reverse: steam hauled then swapped for electric motors.
The motor car codes were coded ACM_ from the mid 1920's to 1931. In 1931 the code was rationalised to AM. The car numbers were 1, 8, 15, 46, 65 and 78.
In 1957/1958, all the AM cars were recoded to straight _M__ coding.
Until more research on the matter, the writer presumes the class was altered as the electric hauled services were abandonned and the cars returned to regular suburban train sets.