An example of what I call a 'flat-face aligned stone pair' [f.f.a.s.] i.e. they align along the narrow face. This makes them very improbable as gateposts, whose narrowest faces are parallel to allow for the necessary fixings, and indeed very few have any kind of modern gate. Mostly are prsently used to hold the floppy "Orkney gate", which is nowadays constructed of barbed wire strung across (usually thin) wooden posts. 'Modern' gateposts using natural stone comprise carved pairs very near to twins. By contrast the makers of the f.f.a.s. went out of their way to choose contrasting shapes (I suppose these could be what is left of 'standing stone' fences, as distinct from the gapless slab fences of the agricultural improvers of the 19thC. Which still begs the question why go to such bother?). Usually only one has an obvious lined socket. And with all the cases I have seen they do appear to be in the right place for dividing up territory.
Of course I cannot prove a prehistoric origin. But the same applies to most of the acknowledged standing stones, relatively few have been excavated or given an appropriate context. Many have tales of being erected in Anglo-Saxon or late mediaeval/ late Viking times. Actually I have a terrible feeling that the Leafea Stone Pair are either a grind ('gate in hill-dyke') or a narrower than usual f.f.a.s.