Firstly, a little local history :)
The Coquet Stop Line, which ran from Amble in Northumberland up the valley of the River Coquet, formed part of the defences constructed to meet the threat of a Nazi invasion during World War II. It was intended to slow the advance of the German army from the north to give time for a field army to assemble on the Tyne Stop Line around 30 miles to the south.
The Coquet Stop Line was made up of a string of pillboxes, with a consistent shape that has been referred to as the "Coquet line type" or more accurately "Lozenge" type, and is only found in the North East and parts of Yorkshire. They tend to have open ground in front, often the river or its floodplain, while to the rear, where the entrance is, efforts were made to provide cover for a retreating force.
There is a further modified lozenge pillbox in Rothbury. It is a standard "lozenge" except for an alteration to the porch and door, which were built on the left hand side as opposed to the rear. The pillbox is well hidden on the south side of the river. It is almost opposite the Whitton road bridge and was sited to defend the bridge. The front three embrasures are just above ground level, with left and right one having been letterboxed. The pillbox has no ID number so it may not be part of the stopline.