The Anderson company filmed a number of special effects and props through the life of Star Trek. This shot, taken a month after completion of the live action shooting, was for the first filmed episode of the third season of TOS, Spectre of the Gun, and shows the set up of the filming sequence. No clapper is necessary as this is silent, so the ID is just to help identify the film leader as belonging to this job but not to synchronize sound.
For other behind the scenes Spectre of the Gun images, see the Melkotian rubber head prop with twinling eyes here:
And the space buoy with spinning apparatus here:
And a very entertaining blooper here:
And a very nice sound clapper of Scotty and McCoy here: