What does it mean?

The M at the bottom right corner could be the initial of somebody's name. It's difficult to tell from the photo, but there might be more engraving at the bottom-left of the cross, perhaps an 'I' or an 'L', making the 'M' the second letter of somebody's initials.

Another idea is that it's an old Spanish acronym similar to the English 'R I P' (Rest In Peace). The Spanish for this is 'E P D' (En Paz Descante for a family member or friend, and En Paz Descanse for more formal burials), with no letter 'M'. But the Spanish for death is muerte so if anyone knows of an alternative, perhaps ancient, acronym ending with M in Spanish, please let us know.

The two stars could mean two bodies were buried in the grave, signifying their ascent to heaven like stars. Is there a Gnostic link? Or are they small Baptismal Crosses?

Finally, the split hoops at the head and arm ends. Do these split hoops represent the breaking of bonds? Is there a link to slavery here? Do they signify the risen Christ?

Do the split hoops represent horns? Horns are sometimes seen on top of a pagan cross, but not on the arms like this one.

Or are they pagan crescent moons as used by the Ottomans and some Muslims? The crescent moon also appears on the Lunate Cross, showing four phases of the moon, understood to have been used by northern European shamans (spirit mediums) for their magic or sorcery. Do you think there is a mystic connection with this cross?