M01a is a random, unscheduled variant of M01, formerly used on the last week of each month on M01 schedules (until 2002).

In late 2017, a new unstable schedule was found.^[2]

M01a has no rigid format, though it is usually a variation of the following:^[1]

463 463 463 50481 50481 [This can be repeated up to 6 times]
111 51962 51962 [This can be repeated up to 6 times]
333 51028 51028
020 18 23
111 999
558 37 = 53827 53827 02213 02213 ... = 558 37
111 51179 53065 [Repeat of groups 15 and 25]
111 000

References

1. ENIGMA 2000 Morse Profiles List [accessed October 3rd, 2014]
2. ENIGMA 2000 Newsletter #110, page 9 [accessed January 8th, 2018]