You are, of course, absolutely correct.
It is a HGU/Artificial Horizon cover for the Canberra T4 aircraft. If you rotate the photograph 90 degrees anti clockwise, you will see how. The two 'L' shaped cutouts fit at two points on the instrument. The top 'L' fits over a specially extended instrument mounting bolt. The bottom 'L' fits over the Fast Erect button.
When fitted, the HGU/AH cannot be seen by the student, only by the instructor.