Hotel room (with cube)

Fifty years ago today, July 7, 1966: Keir Dullea and Gary Lockwood had just completed shooting their scenes and left England; before diving into the 18-month period of perfecting the special effects scenes, Kubrick and the crew had some pick-up shots to do. This is the last day of shooting in the hotel room "with cube" - everybody still called the monolith that way, despite the fact that it had a slab-like form since late 1965.
It is unclear (to me) if the "matte shot" on the clapperboard refers to a visual effect that was meant to appear ON the monolith (an idea that had already been ruled out in April 1966 in relation to the 'lessons' the 'cube’ was to impart to the man-apes) or, most probably, to the shot of the bed where the Starchild would eventually appear.