 Complete the following phrase:  Four car units operated on the Chesham shuttle and, from 1977, on the East London line. The transfer of COCP Stock from the Metropolitan to the District line in the early 1960s allowed some of the Q stock to be scrapped. In 1968, an order was placed for 35 six car trains to replace the COCP Stock on the Hammersmith  City and Circle lines. These were arranged as two units with a driving cab in the motor car at one end only and normally run as three pairs. These trains were designated C69 stock. The remaining COCP and R Stock on the District line were replaced in the late 1970s by new trains. A shorter train was needed on the Edgware Road branch due to the platform lengths so more of the C stock units. The rest of the District line could use longer trains and new D Stock trains were introduced between 1979 and 1983. A new Fire Safety Code of Practice, drawn up following the 