The smaller stone here has been so regularly shaped that it would be impossible to tell if it had a prehistoric origin. The larger stone is a difficult one to call as far as I'm concerned, it may be prehistoric but has been tooled on one face and at least one side with the face bearing an inscription from 1784. The barrow is also a little suspicious due to it's size and the fact that although it's marked as tumulus on the map it doesn't appear in my copy of the SMR.
Scratch head and file under Hmmm....
The NMR records "a boundary stone shown on an ordnance survey map may be of prehistoric origin. It was apparently used as a boundary marker from the early Medieval period onwards. The stone was located by Stanhope Whites survey".
In my book Stanhope White was a giant of a fella, he worked with the Leakeys in East Africa.
After he retired he dedicated himself to traipsing the moors recording everything he found, including the many hundreds of standing stones, ancient and modern.
If Stanhope judged this stone as possibly prehistoric in origin, then that's good enough for me.
There are actually two stones at Flass Brow both of which have been carved to some degree and both bear inscriptions. I think the larger of the two is Stanhope's stone.
I should also add that the OS has this spot marked as a tumulus. If it is a mound it's a low one but kinda fits in with the NYM tradition of placing boundary stones ontop of prehistoric mounds.