You're welcome Don - Gary's Mustang cockpit is bang on for a WW2 era aircraft, and there's loads of pics in his thread. If you need any more, let me know, and I'll dig out some for you.
No Don, that 'pan' under the seat is probably the relief tank.
The rectangular cushion, on the vertical back rest of the seat, is what I mean. It shouldn't be there for a WW2 ETO Mustang - it should just be the semi-dished seat back. The pilot of the Mustang (all models) sat on a dinghy pack, in the well of the seat pan, and his back rest was the large, B-4 Type, back-pack parachute (the opposite to the P-47 arrangement).
As far as I know, that rectangular cushion was also a flotation device, and it was yellow in colour, and was a post war addition, when I'm guessing a change to a seat-type parachute, with dinghy pack incorporated between parachute pack and harness, was made. It might have been used in the PTO, but that's not my area of 'expertise', for want of a better description.
Unfortunately, most model and accessory manufacturers base their products on current, preserved aircraft, mainly museum exhibits, which seem to have the cushion, and faithfully replicate what they see and measure on the 'pattern' aircraft - including, sometimes, such things as repair patches, replacement (and non-authentic) parts, and quite often, fully extended oleos, where displayed aircraft have been on axle jacks to relieve the weight off oleos and tyres. Faithful reproduction, but if a few questions had been asked at the time of measuring and planning, more period accuracy would have been achieved.
I have seen accurate resin seats without the cushion, but if it can't be removed, then paint it the same colour as the seat (Medium Green) and live with it.