From then on the Finns had to defend their freedom from the newly established Soviet Union. The need to keep an eye on their powerful neighbor forced them to show special interest in radio interception and cryptanalysis.
Decoded messages can be found in the Finnish national archives. Turkish and US diplomatic traffic is in folders T-21810/4 and T-21810/5. These include intelligence reports from Bern, Switzerland.
Soviet military 2, 3, and 4-figure codes were solved regularly. The 4-figure OKK-5 General Commander’s Code was captured during the Russo-Finnish war of 1939/40 and many subsequent 4-figure codes were solved.
It seems that their solution of this system depended on German help. According to several sources they received US strips and enciphering instructions (almost certainly of Japanese origin) from the Germans in 1942.
Pics: flag from wikipedia, M-138 strip cipher from wikipedia commons user Raul654 , decoded messages from British national archives HW 40/132 ‘Decrypts relating to enemy exploitation of US State Department cyphers, with related correspondence’ and HW 40/29 ‘Exploitation of Russian Civil communications by Axis Powers’.
Additional information: For examples of decrypted messages check Decoded messages in the Finnish national archives.