Visiting Devizes Museum's wonderful "The Splendour of Stonehenge" exhibition I noticed this detail in a pen and sepia wash plan of Stonehenge by W H Hyett dated to 1820.
Around the corner there is John Britton's Celtic Cabinet believed to date from circa 1824 with this model by Browne in the top
. Again here is a detail.
Stone 156, the fallen lintel of the Great Trilithon, has its mortice holes upwards, whereas today it lies on its side with the holes on the vertical faces.
On the side of the cabinet there is a plan of Stonehenge which also shows the mortice holes upwards;
But, of course, today the lintel's mortice holes lie on the side of the stone.
Did someone move the stone?
Judging by how the model's lintel lies against the Bluestones it looks as though it has been rolled over.
Browne made other models and it seems in at least one of them -http://mikepitts.wordpress.com/2012/05/15/whats-that-hole-in-soanes-model/ - the stone is lying in its present position. We also have Stukeley's earlier drawings which also show the same.
So it looks like it the lintel was rolled over, towards the south west, in the early 1800s and at sometime it was rolled back.
UPDATE: June 2016 - I have noticed in John Woods 1740 book Choir Gaure it is described as a "Rocking Stone" which can be moved be the force of a mans finger, which it certainly isn't now.