In the post below http://www.sarsen.org/2012/06/twisted-56-and-altar-stones-and.html I discuss the significance of the twist to Stone 56 - Gowland who did the meticulous re-erection and excavations of the stone also excavated under the fallen Stone 55 and concludes it was in line with 56. So it was a twisted trilithon in line with the midwinter sunrise.
Scans of his report are below, click on them to enlarge them to readable size.
For a discussion of the position and Cleal's note on it see http://www.sarsen.org/2012/11/stone-55-stonehole-correction-to-cleal.html
More at http://www.sarsen.org/2014/06/stone-55-alignment-and-original-position.html