Not quite Steph.
Those pics show what may be a yellow cushion, or perhaps a version of the flotation device back cushion, along with the original US seat harness.
As mentioned in my reply to the post by Wojtek, in the video it looks like there is no back rest cushion, unlike the earlier RAF Mustang Mk 1 etc variants, although this is not certain. My feeling, based on what is visible in the video, is that the cushion was not fitted ( the back pad on the British seat-type parachute assembly was padded, albeit lightly, and this, along with the "Mae West" life jacket and flying clothing, was normally adequate to serve as comfort support).
Also, in general, American aircraft in RAF service had modifications carried out at MUs, fitting some "standard" British equipment, including seat harnesses.
Although it's
possible that the original US seat harness was retained (as shown in your photos), most photos of RAF Mustangs show what appears to be the standard Sutton harness. with colour photos suggesting this was the case, judging by the colour tones and perceived textile textures.
From what I've seen in photos, it is possible / probable, that this harness was the later version, with the quick-release box, as shown in the photo below.
However, for ease of construction, and given that you may use the 3D printed seat, I would suggest painting the seat back cushion in a mid brown shade, and altering the lap straps of the harness to represent the Sutton harness. If using your seat previously posted, then omit the cushion and either adapt the aftermarket seat harness, or make one from scratch.
Note that the harness shown is configured for a Spitfire. For the Mustang, the shoulder straps would be configured as per the seat pics you posted.
Also, note the darker colour of the Sutton harness.
Hope this helps.