A Combined Cipher Machine (CCM) was created for the US and UK to use for a variety of purposes in 1941, when the two countries were shipping resources across the Atlantic together in convoys.
The British were not impressed with the US’s M-138A and M-209 machines, while the US were not forthcoming with their M-134C machine, seeing it as too secret to share.
The CCM was developed as a compromise, starting with a British Typex machine and a US Navy officer’s attachment of additional rotors to convert the Typex into the CCM.
There were 8,631 of the machines made by the end of 1943, at a cost of $6 million each, but the machines had a problem of occasionally having a very low cipher period.
