Baker was born in 1898 in Vukovar, Kingdom of Croatia-Slavonia, Austria-Hungary (modern-day Croatia), probably under the name Rudolph Blum.
The identification comes from coded correspondence between the Comintern and the party in Russia's RTsKhIDNI archive (495-184), which contains dozens of messages from BROTHER in Moscow to FATHER and SON in the United States.
Annotations on these messages identify BROTHER as Dimitrov and FATHER as the party General Secretary Earl Browder.
In these messages, SON is the head of the party's covert arm or secret apparatus, which Baker took over in mid-1938 after the defection of Whittaker Chambers.
[citation needed] In May 1942, General Pavel Fitin, the head of KGB foreign intelligence directorate states in a message to Dimitrov found in the Comintern Archives, "We are forwarding a telegram we received from New York addressed to you from Rudy"; this telegram is signed SON.