The object is shown from the other side in
NS4759 : Small marker, Brownside Braes, where links can be found to images of other such markers on these moors. They are not very big: the top is about 10½" square.
(I had originally taken them to be orienteering markers, but I am told that they are most probably markers that were located here in connection with the nearby reservoirs, and that it is not unusual to find, in such settings, ones that are unlike the standard OS trig points: see the comments at
SK9408 : Not a trig point.)
In the background of the present picture, on the left, is Glenburn Reservoir. The skyline clump of trees in front of the pylon on the right was the site of the long-gone farmstead of Duchallaw. A dark feature on the slope in front of those trees is
NS4759 : An old quarry in a field.