Looks like he standard, NA seat, but with the pad fitted, to prevent wear / abrasion to the rip cord housing for the seat-type parachute pack..
American parachutes (back pack for the Mustang) had the ripcord 'D' ring on the left shoulder harness, with the cable tube running over the left shoulder to the pack.
What is interesting though, is the fitment of the British Dunlop grip on the control column - far as I know, and confirmed by a RAF pilot I knew (now deceased), the later P-51B/C and D/K retained the American 'pistol' grip in RAF service.