I guess the guitar is a Spectrum, based on the location of the toggle switch.
He's pictured with a 12-string Spectrum (must be a custom as they aren't listed in any catalogs and I've never seen one for sale) on the cover of a guitar magazine from 1987 and also inside the Big Generator tour programme.
I wonder if the photograph was taken during Yes rehearsals. Or, could it have been done at St. Louis Music while he was visiting there? Has anyone else seen any pictures of TR or Yes in front of a chalkboard?