I think the Afro was bigger. Much bigger.
In his Guitar Player interview in the late 70s they recounted his first use of an Alembic. He had been playing an EB-2, which may have been a 2D, and the Alembic rep who saw him live told him how it sounded like "shit" and he should try out an Alembic. Stanley seems to have agreed somewhat, as he seems to have kept the Gibson but did switch to the Alembic. And much like the shift in bass tone that graced Led Zep discs after JPJ got his Alembic, Stanley marched headlong into nasal clanky-clank land. I am no fan of fusion, but even so, I wish the guy had kept a Gibson around for at least some of his recordings. So, maybe he did have a G3, as it is possible the fine folks at Norlin must have read the article and thought maybe they could win him back with a freebie. But I don't think there is much real evidence of him playing one. And that pic doesn't look real.