The dry stone wall structure shown in the 1906 photo & description is suggestive of a butt,. I agree they do not usually occur singly but if you look at the OS map a line of butts is shown extending up the hill on the opposite side of the valley. Could the structure in the circle not be an extension of this line with the intermediate ones drowned in the reservoir?

Maybe a look at the butts remaining on the hill could cast some light on things, Looks like I’m going back again in the next few days!