In my essay German intelligence on operation Overlord I said about the M-209 cipher machine:The M-209 cipher machine was used extensively by the US armed forces in the period 1943-45. Army units in England sent training messages on the M-209 which the Germans decoded.
The USAAF used it in operational and administrative networks.M-209 traffic together with D/F may have allowed the Germans to discover the concentration of US forces in the South.
After having a look at the report E-Bericht Nr. 3/44 der NAASt 5 (Berichtszeit 1.4-30.6.44) it is clear that the Germans were in fact able to get order of battle intelligence on the US forces in the UK. In pages 2-3 it says:
Activity report before the invasion…………………………………………
1). AM1:Focused on decoding the AM1. Ten absolute settings were recovered, which brought the deciphering of 1,119 messages. This cipher-material, mostly composed by the U.S American Expeditionary Corps, gave valuable insights into the location of enemy groups.
AM1 (Amerikanische Maschine 1) was the German designation for the M-209.