The book ‘Hitler, the Allies, and the Jews’, p265 says about Mayer:
The same book mentions an OWI message from Mayer to Elmer Davis, dated 25 May 1944, decoded by the German codebreakers on June 6. The telegram ‘described the holocaust in Carpatho-Russia and the Mamaros areas of Hungary on the basis of ‘’reliable sources and even on the basis of Hungarian newspapers’’. From the contents of the decode it is clear that it was OWI telegram Bern-Washington No 3.346:
The Finnish codebreakers also read these messages, as can be seen from decodes found in the Finnish national archives (the originals can be found at NARA, collection RG 59):
Bern-Washington -28/1/1944 - No 564
Bern-Washington 9.3.1944 No.1438 (to Elmer Davis OWI from Mayer)
Another report found in SRH-145 ‘Collection of memoranda on operations of SIS intercept activities and dissemination 1942-45’, dated 16 October 1943 says that the OWI was using cipher teleprinters with one time pad tape, Hagelin cipher machine (presumably the M-209) and double transposition.