M usually stands for "manual" which means no automatic settings. Normally that is what you set it for when you are going to control the stitch width and length yourself. So basic sewing and zigzag. Keep in mind, back then "embroidery" was done using one of those two - and not a programmed or cam driven stitch. When D is used for "darning" it is most often a feed dog setting, as you would lower the feed dogs for darning. I've got several machines that have "darn" as one of the setting for the feed dogs (Darn, Silk, Normal).
D could easily stand for DISK - as very often they are referred as pattern disks instead of pattern cams.
Sounds like you have a machine that takes disks, and has one built in stitch (blind hem) along with the manual(zigzag). Remember, zigzag width set at 0 is a straight stitch, so on these machines, straight stitch is a manual zigzag setting.