in this recording of *AT-312.5 you
hear several times the callsign, and a long dash for direction
finding (rb, 2013)
see here the very detailed transmission scheme from the german
nautical officer's handbook of 1951
a PPC from the files of Patrick
Robic (2001)
------------------------------------------------------------
Callsigns:
AT
U
Frequencies Morse Telegraphy:
312.5
318.5