I've seen this with other quarries too. There were also wrought steel bracket frames that clasped a stone - two were used - and allowed a gate to be hung from it. Wales in the 1960's had loads, they're also sometimes seen in Northumberland. When the gate and the hangers are gone few would ever know that the stone once hung a gate.
The rectangular holes in ancient standing stones are susceptible to analysis by the Megalithic Inch.
