M13b was a variant of M13 with messages that were up to 70 groups long, longer than the average. However, if a message 23 groups or shorter was to be transmitted, the M13a format was used instead. The format was otherwise identical to M13.
M13b had a schedule on 2nd and 4th Saturday and Sunday at 21:00 UTC, or 22:00 UTC during European winter time.
ENIGMA Control List, Revision #25, p. 18 [accessed October 7th, 2014]