|Known counterpart stations||E07, G07, S07, V07, XP, XPA, XPA2, XPH|
|3-figure transmission ID|
|Repeated for 2 minutes|
|903||903||903||1||8603||63||27073 15164 79261 ... 48334||000 000|
|3-figure transmission ID, repeated thrice||Amount of messages in the transmission||3- or 4-figure group, purpose unknown||Amount of 5-figure groups in the message||5-figure single groups|
|Repeated for 2 minutes||Repeated twice|
The intro and the outro are sent at 17 WPM, while the rest depends on the schedule - anywhere between 22 and 30 WPM. Zero is sent as "T" - a single dash. M12 with more than one message is designated M12a.
M12 operates in regular schedules of three identical transmissions, spaced 20 minutes apart. The first transmission can take place at any 10-minute interval within an hour, but most commonly it's either the full hour or half hour. If the first transmission took place before 12:00 UTC, the frequencies of the subsequent repeats increase - otherwise they decrease. If the transmission is too long to fit into the 20-minute frame, it continues, then the subsequent repeat is started as soon as possible.
M12 has two types of schedules - those with the same ID, frequency set and time throughout the year, and no null messages - and those with ID and frequency set changing monthly, and the time adjusting for daylight saving. On the latter type, the transmission ID and the frequencies used are correlated - putting together the hundredths digits from the frequencies, from first transmission to last, gives the ID.
It is unknown when M12 was first noted, but it was probably in the 1970s. Its operation habits have not changed since then.
Prior to early 2000s, the null messages and the intro were 5 minutes long, rather than the current 2 minutes long.
In July 2011, M12 was heard with several unscheduled messages exceeding 1000 groups, which took over an hour to send at 30 WPM.
Starting mid-2014, M12 is experiencing technical difficulties on the first scheduling type, including breaks in keying, variable audio level at the beginning of a part of the digits. The second scheduling type is so far unaffected.