The mixed “VM” appears on lots of Peak District gritstone from the 17th and 18th century. Of the possible explanations three seem to be the most probable. The first is that it is a Catholic sign for the Virgin Mary in the era of the Reformation, when persecution of Catholics was rife following the Glorious Revolution of 1689 when James the second a Catholic sympathiser was overthrown.
Practicing Catholics marked boulders and buildings with the ‘VM” inverted or otherwise to signify places where people gathered for worship. Which may fit with the markings on the gate post that sits in a dry stone wall above Marsden near Standedge Tunnel. Close by is an old farm with mullioned windows from the same period.
It could also be used to signify supporters of the protestant William and Mary who became joint monarchs in 1689 following the Glorious Revolution. The use of the intertwined WM was seen as a denouncement of the Catholic VM. As may well be the case with the markings at Stanedge Pole near Stanage Edge in the Peak District. Or perhaps these signified Catholic markings and were connected to the Catholic Chapel at Padley where the priests were caught, hung drawn and quartered and became Martyrs in the process.
Of course it could just plainly be some ones initials and they liked chiseling it on to gritstone.