The Prefixes "DA...DM" are valid from January 1949 until September 1973
for the
common DXCC GERMANY !  (includes East- and West-Germany) 
F
ederal Republic of Germany (9/48 until 10/90) and German Democratic Republic
  (10/49 til 10/90).
From 1952 until 1979 "DM" was the mandatory prefix for radioamateurs in East-Germany.
September 17th 1973 the GDR became a Member of the United Nations.
From that day until December 31th 1979 the prefix  DM/DT counted for a new DXCC-country 
in the ARLL-DXCC-List.

Later on as a political mark off  GDR changed the prefixes DM / DT into the new prefixes Y2...Y9.  Until the reunion, October 03rd 1990, the Y2 ... Y9 prefix was valid for a seperate DXCC-country.
Starting with the Unificat on Day, the ex-GDR-operators changed their callsigns into DA ... DL-calls from the FRG.  I got now the new  "FRG-callsign"  DL7UDO.
But, my most favourite call is still my first individual callsign
DM2FDO.
Because I had to work hard to get my licence ... this call is printed on most of my awards and...
i come from GDR. No mask my past with a DL, DJ, DF-call !

In 1998 it became finally possible to apply for my old ex-DM call  DM2FDO.
I am one of only (abt) 175 ex DM-call-owners work on HF-Bands in Germany.
Oldtimers callsign-identification:
DM2xxx, DM3xx to DM6xx or DM3 to DM6xxx.

Since 15. 08. 2003 also licence class 2 OPs (former only U-, / V-, /SHF) an Shortwave bands with new DM-callsigns (DM1xx, DM7xx, DM8xx, DM9xx). DM is now a normal Prefix in Germany.