Found the other end of the possible cist. The distance between the orthostats inner faces is 1.56m and they are offset, the south end of that at the right being in line with the centre of the left-hand one. Measurements are as follows :-
LH 0.56x0.08m, protruding 0.12m from the soil at either end and 0.27m in the middle. RH has disparate edges, the whole being 0.46x0.15m protruding 0.13m, the reduced face 0.33x0.05m showing 0.08m.
The latter, also has a stone lying down against it, only 0.6x0.26m compared to the other's 1x max. 0.48m. But there is at least one other stone between the slabs so they would appear to date from a later period rather than being co-eval.
It seems curious to me that this mound is in such an odd-shaped 'field', even more so as this is only half of larger section of similarly strange shaped 'field'. Though the eastern half appears fairly level, but I do wonder if it too once produced archaeological finds, as in Orkney farmers often keep odd pieces of land out of cultivation because at some time in the (often forgotten) past there was a reason for so doing - only occasionally do we find what site was at these places.
Coming into Deerness, at the beginning of the last leg of the A960 before it becomes the B9050, two minor roads at right angles head for the coast, one to the Newark slipway and the other marked as coastal path and geo (also called a goe in Orkney, a deep gap in the cliffs resulting from a cave's collapse) - ?Muckle Castle. Between the two endpoints was a non-burnt mound WSW of Mussaquoy and beyond the coastal path. Go down the slipway road and turn right and walk a little way to a strangely elongated field containing the site. Even from a distance you could tell it had been excavated, end on looking not unlike a settee in outline if you ignore the grass. The O.S. map of 1882 only shows a narrow stretch of rough land. And I wonder if we now only see part of the rim of what was a much larger structure a long time ago - close by is a feature called Peerie Castle, were they once one?
The entrance to the field was an extraordinarily taut 'Orkney gate', thus requiring a push from distance leaning slightly as it could well be risky to attempt from close and straight on. This site is a narrow oval mound with a long rectangular pit taken from the (NNW?) side. Unfortunately a low battery stuck my camera's zoom on telephoto. Though there were two edge-set slabs at the front of the mound I can see only one, at the northern end. But this may be owing to the time of year. However I see a long stone prostrate beside it. Scraping away the thin grass covering reveals your usual standing stone shape of a few feet long, which is brown in colour like you would see in an old house (there is some in the exposed material too). There are red stones in the vertical section behind this and red fragments in a smaller exposed section at the southern end of the seaward side (ESE?), however this seems to have originally been structural and is in no way burnt. Perhaps this fell in from above when this place was 'quarried'. However the stones over this mound are a heterogenous lot anyway, differing in size and shape and colour, and including ones similar to the prostrate stone.
To me it looks as if the site could well have been adapted from its original form/purpose even before the quarrying. The earthfast stone pair have been seen as perhaps part of a cist, but equally well fit the original burnt mound theory as resembling the 'tanks' of a site such as Hawell. I don't get a feeling either way and distrust both interpretations here. It is a very curious place, nothing adds up, the individual parts don't make a recognisable whole.
An S.W.R.I. talk in 1936 mentions the same site ["The Orcadian" 17/9/36]. Coming from the Newark direction, a short distance from reaching the Peedie Castle rock stack two graves found in the mound Howe Hurcus "at one time" led to the attempt to dig it away being abandoned.
So perhaps two short cists rather than a long cist.
At various times RCAHMS NMRS record no. HY50SE 1 at HY56590369 has been seen as a burnt stone mound and a turf mound with burnt earth, but is no longer believed to be any kind of burnt mound. It has steep sides and is thought to have been circular until the second quarter of the 20th century when the owner 'quarried' the west end, leaving it slightly crescentic. He found two foot-high earthfast stones about five feet apart and some large stones besides. It now measures 10m by 12m and stands 1.2m high.