Emission mode: MCW, ICW
Believed Country of Origin: Unknown
M13 has many variants and commonly sent messages in ICW Modulation but sometimes MCW. M13 used a 3-Figure ID (###) with 5-Figure message groups at 9 wpm and used a short zero.
M13a was a variant of M13 that repeated the ID 3 times
M13b was a variant of M13 that sent longer messages up to 70 Groups and changed its ID monthly.
M13c was a variant of M13 that that sent a slower message 2 times a month.
M13d was a variant of M13 that sent messages on 5876 kHz and 6715 kHz
only at 0230z. Format followed: Preamble: NNN x5, BT, Message Number,
Group Count (##), BT Message:### x ## BT ### ## BT, with the longer
message format ending with BT ttt.
M13e was a variant of M13 that changed ID’s every month and sent 3 messages every day at 0600z, 1600z, and 1800z.