As it had no effect on mileage earning (M = 100%) I am not sure that it stands for anything. All of my KL award flights booked in the past year showed up in my FB statement as "M0" [although of course M is not the booking class that economy awards are made in] so I guess it is just a way to allow the system to know that this is an M for which miles should be awarded.

Have you any theory as to why they may have not recognised the ticket correctly? Anything unusual about how it was purchased or any disruption while travelling? Was it operated by a partner?