Doesn't seem to have been, but by 1850 the capstone was no longer supported by 5 uprights, so it seems likely to have been some time between 1824 and 1850.
But. I've no idea what angle the capstone was at in 1824.
You'd think it would have been recorded/reported wouldn't you? and do you find it strange that all but one of the stones at Trethevy stand on the ground not in it? this always seems very wrong to me when i'm there.