I had been along the old Finstown road several times going to Kirkwall and so was quite surprised to see a rectangular hole in the RH road verge (HY391118). Closer too it resembled more a cist formed of thin slabs set against one of those broad patches of turf that cross ditches to let farmers onto their fields. When I came to this ?tank I saw that it actually lay in the midst of two such bridgings, each 5m across. Kinda weird. The feature appear sub-rectangular because the edge of one slab has moved forward slightly,it would have measured 0.9 by 0.8m and is 0.5m deep. To the naked eye the 'tank' is certainly a little forward of the ditch line but a pic doesn't appear to bear this out.
On my second visit it took me walking twice over the ground before I found it again. Now that the water-levels are much lower it has more the appearance of a well than a cist, the full depth 0.9m (as far as I could find) with what I took for the bottom being ledges on two sides. The ledges make you think of corbelling but there's only one rough-edged ?slab on the left extending as a near triangular shape 0.25m along and 0.6m from the back left corner, with drystane walling above, and one on the right a slab 0.45m long and only 0.1m from the back right corner. Apart from the section of walling and the 'ledges' the structure is of slab construction (including a triangular stone making up the right back). The base is hidden by a couple of short bits of wood and a thin layer of other rubbish. Almost feels like part of a souterrain, certainly something of at least that age.
P.S. revised my opinion back to its being a short cist (?double?) on finding that one of those at Queenimoan had an erect stone wholly buried at one end. Ending the enigma this is my best fit.