Updated November 2012
I've been contacted by Angie Lake, who's been researching carvings on Stone 16 since 2006. She doesn't see a spiral here, but has detailed her own discoveries and thoughts over on the Megalithic Portal website.
This image on the Megalithic Portal website shows the features Angie has identified on each of Stone 16's faces.
Angie's profile on Megalithic Portal.