Holed stones, like the one is this picture, were traditionally used on Dartmoor as a means for fixing gates to stone walls. However, the holed stone was always placed along the top of the wall - never as low down as the one in this example.
Presumably in this case somebody has re-used a redundant relic from an old gateway in a new wall.
The photograph was taken on 16/06/1935.
Image - Glass Plate