Cf Pattison's "Avebury's Stones" book p 80
This right profile face appears to be a simulacrum and not a carving (although I do sometimes suspect that the horizontal line through the "eye" might have been an artificial addition over a natural roothole already in the stone). Its also worth pointing out that the reverse side of this stone has definitely been artificially carved with a crude artificial face to partner this natural one - (see this by clicking link 1 below).
The specific features visible on both sides of 29a teaches us exactly how the Avebury monoliths in general were selected, erected and carved. Click here (link 1) both to read my explanation of this process and to view the reverse side of the monolith. Click here (link 2) to read a note concerning the possible celestial influences behind the creation of this monolith.
Image copyright David Baldwin Night Photography