Hidden in the shade is the investiture relief of Shapur I (241-272), the son and successor of Ardašir, who had ordered the first relief at Naqš-i Rajab. It shows his inauguration (on 20 March 242) and is inspired by the investiture relief of Ardašir at Naqš-i Rustam. The difference is the absence of defeated enemies, which proves that it was made before Shapur repulsed a Roman army. On other reliefs (e.g., Bishapur I), we can see a dead Gordian III and a Philip offering his submission. The relief opposite the Investiture Relief, the Equestrian Relief, has about the same age.
So, we can only see how Shapur receives the ring of power from his god. The faces of the two figures have unfortunately, long time ago, been damaged by someone who obviously did not like Zoroastrianism.
The same theme was used by Bahram I in Bishapur, relief IV.