My latest wild-haired theory is that the current altar stone was brought with the Bluestones, and that another alstar stone would have been coupled with the sarsens. If a well-known sacred stone existed elsewhere it could easily have been donated to fill the need for an alternative type of stone for the altar. The sarsen circle, complete with Cotswold Limestone altar, later added to by the bluestones with the Milford Haven altar.

Recumbent stones are often of a different type to the rest of the circle, so why not the Altar?

My Castleden's book arrived this morning.. he says south stone is only 0,75 m wide and only 0.22m the northern stone being shorter... it could be a single longstone (as proposed by Engleheart in 1933), which may have been similar to the altar stone, but it could well have been an Avenue stone.. The Engleheart theory was written in article form in the Wiltshire Gazette. Only putting the avenue stone theory because theres a heavy argument in favour of "altar stone" whereas it could have come from somewhere else (I do like an argument to be balanced ;)