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?