This is still HM01, just a probable error. Pretty much every Cuban numbers station has been seen in LSB at one time or another, and it is not all that uncommon, you see them do this several times a year. USB errors have also been seen, but are uncommon.
Cuban numbers station M08a (the Morse code sister station to HM01 and V02a) has two major modes of operation, MCW and ICW. MCW is full Double Sideband with carrier (normal old AM) and using a 1 kHz tone to make the dits and dahs. So it has a carrier and can be received with a regular old AM radio. The ICW is interrupted CW. The Cubans appear to use two different methods to get the ICW. One is just keying a carrier off and on, just like any other CW station does it. That technique appears to be used less and less by the Cubans. The other way to make ICW looks / sounds like regular CW with the RF keyed on and off, but it is not. The Cubans do not key the carrier, but rather use the transmitter in a sideband mode and the same 1 kHz tone as they use with MCW. They typically use the transmitter in LSB mode to do this.
Most, but certainly not all, odd LSB HM01 and V02a transmissions seem to follow M08a ICW transmissions. The Cubans are probably leaving the transmitter in LSB mode from the previous transmission and then starting what is supposed to be a full AM transmission. Indeed I have recordings of such transmissions starting in LSB and several minutes into the message switching to AM. One time I saw them start in LSB for several minutes, switch to USB for a minute or two, and then end the transmission in AM, they covered all bases that day
M08 is not the only numbers station that uses an SSB transmitter and audio tone to make CW, the Chinese station M95 does the same thing at times, and one of the Russian M stations uses USB in the same way. Sometimes you can tell such transmissions by the appearance of an image / audio harmonic CW signal. An example would be with the transmitter tuned to 8137 kHz in LSB, a 1 kHz audio tone would make CW that appears to be on 8136 kHz, and there might be harmonics of the audio dits and dahs at 8135 and possibly 8134 kHz.