Thank you.
It is a rectangular hole in the bulkhead panel with a new panel riveted into the hole. The new panel is to allow the FIA regulation sized suitcase to be fitted into the trunk. The 289 race cars had protrusions in the trunk lid for the same reason. This was part of the FIA GT homologation requirements in force at the time.
The new panel protrudes into the cockpit by about 1½" at the top edge, but tapers down to nothing at the lower edge.
It doesn't provide an benefit or functionality, but I just liked the extra originality feature of it.
Paul