In the site primarysources.brillonline.com there are several decrypted Greek Communist military radio messages dated 1948.
These messages were deciphered by the US Army Security Agency.
Unfortunately no NARA reference is given but from the notes in some of them it seems they can be found in collection RG 38
1). U.S. Army, Report, Greece: Move of GHQ of the Greek Democratic Army, March 4, 1948, Top Secret Glint
3). U.S. Army, Report, Greece: Thessaly Hq of the Greek Democratic Army, April 3, 1948, Top Secret Glint
4). U.S. Army, Report, Greece: Agent Traffic on Greek Guerrilla Network, May 20, 1948, Top Secret Glint
6). U.S. Army, Report, Greece: Rebel Supply Depot Probably in Bulgaria, June 8, 1948, Top Secret Glint