Yes. In one of your photographs there is what appears to be a dinge in the underside of the capstone. Toward the mid-line, form the aperture. When I look at other pictures the lump and the round depression seem on completely different planes. But after looking at the two links that Tiompan posted, to other dolmens, then it seems obvious which upright pointed stone has been used to support the capstone and to provide a pivot. I guess our ancestors were predominantly right-handed, as we are, and that would be the easiest direction to shift it into place (anticlockwise). I need to read some online material about these structures, but am presently researching prehistoric gold-working techniques. (Dem hair rings!)
It looks as if both pointed and flat stones are used for portal ,side back and doorstones with no obvious pattern that I could see . Although it is not do with handedness it may be of interest that Ms Kytmannow writes " there is no indication of any emphasis of a more dominant right side of portal tombs ......The heights range from 0.5m to 3.5m on the right and and from 0.6to 4.4m on the left although the 4.4 example is out of it's socket and would have probaly stood at 3.5 . The laerrgest still up0right is at harristown 3.6 and 3.5 respectively ." http://www.themodernantiquarian.com/site/2305/kilmogue.html
The side stones at Trethevy are cleverly positioned and overlapped. The capstone is supported at three points, on the closure orthostat and the two rear flankers and between them secure the other two flankers. So what you have is the rear flankers secured by the capstone and the front flankers underlapping them and prevented from falling in by the closure stone which they lay against. They have no connection with the capstone whatsoever. Very clever. It is this 'lack of contact points' that actually prevents the capstone from sliding off as the downforce on those three points is much greater that if it was multipley(sp) seated.
It has been argued that the tripod tombs e.g. legananny http://www.themodernantiquarian.com/site/1055/legananny.htmlshow that sidestones were optional with dry stone walling , soil or timber used to delimit the chamber . Of 217 tombs 132 sidestones survive on the right and 121 on the left .,a second sidestone is present 19 times on the right and 14 times on the left .