M15 was ran by German Federal Intelligence Service (BND). It had a legitimate callsign DEA47 and transmitted on 12283 and 13582 kHz in parallel. Most of the time it sent a channel marker in fast, automated Morse - "VVV de DEA47", but sometimes it sent short messages - however, the format of them is now unknown.

For many years, it had a bogus callsign EC3Y, and transmitted on 9161 kHz. It changed to DEA47 and new frequencies on 20 January 1992.

References

ENIGMA Newsletter #7 (January 1995), p. 31 - read a scanned version here