I'm not sure they did use any kind of cipher. My understanding is that they communicated primarily through ambassadors and diplomats, who would use their own ciphers and methods to communicate back to their home countries. There was no direct line between the governments of the Soviet Union and the USA at that time.

In other words, if the US had a message to send to Stalin, they would give it to whomever was the Soviet ambassador to the US at the time. The diplomats would encode it and send it over telegraph back to Moscow.

(There was a direct phone line between the US and the UK, that did use an encryption device to scramble it, but this was a unique creation during the war and very unusual. Supposedly it made Roosevelt and Churchill sound like Donald Duck.)