By 1973 models were being made with both M4 and M8 clocks, both with beige thumbwheels. The M4 was used in the 855c and 855d. The M8 was used in the 853a, 854a, 855a, 854b and 855b.
In August 1974 it was agreed to continue making the M4 clock until the end of 1974. At that time they were being turned out at a rate of 4000 per week. Production would slow to 1000 a week over these few months, with workers being transferred to other products as required.
In August 1974 production of the M8-C model was in progress.