This scene, in the centre, shows how Ardašir's son Shapur unseats Artabanus' vizier. Note the fine armour of the rider and the horse.
Picture Source: allempires forum
Finally, the fight between the two kings: Ardašir defeats Artabanus, whose horse turns a somersault. The victorious new king attributed his success to the god Ahuramazda, which is the subject of the second relief.
The relief shows three stages in Ardašir's fight against the Parthian king Artabanus V in 224. The Persian leader, who had united the southern part of modern Iran, had built Ardašir Khureh, and Artabanus was worried about Ardašir's increasing power. When he arrived with an army, however, he was defeated by his rebellious vassal and the Persian became the new "king of kings". This was the beginning of the rule of the Sasanians, which was to last until the rise of Islam in the seventh century.
Main Source: livius.org
Firuzabad or Firouzabad (Sassanid Middle Persian Ardasher-Khwarrah, since the 10th century A.D Fīrūzābād ) is a city located in Fars province south of Shiraz.
Referenced as figure 50 in The military technology of classical Islam by D Nicolle
50A to 50C. Rock-relief, Triumph of Ardashir I, early 3rd century AD, Sassanian, in situ, Fīrūzābād (Rob OA, Pope).
See also A Sassanid Persian Clibanarius in The Armies and Enemies of Imperial Rome by Phil Barker & Ian Heath, based on the Rock Relief at Firuzabad
A Parthian Cataphract in The Armies and Enemies of Imperial Rome by Phil Barker & Ian Heath, based on the Rock Relief at Firuzabad