GB-64 1/48 P-51D - One Trick Pony (2 Viewers)

[Airframes]

I think that may be the parachute pack, albeit lower than I would expect. Note what looks like the pocket for the "D" ring ripcord handle (arrowed), although I can't see the handle itself, or the flexible tube housing the ripcord cable, which ran back over the shoulder, which may be hidden by the oxygen mask.
As it seems that the kit part has the cushion moulded onto the seat, I wouldn't worry about it, and just leave "as is", as it is possible a seat cushion was fitted in this instance, and maybe a change to a seat-type parachute pack, as used with the P-47.

 

[Crimea_River]

There's a build thread over at Britmodeller with excellent reference pics and discussions.

Missouri Armada P-51D Mustang: documents and partial scratch from the Tamiya 1/48 kit

Hello to all, After the amazing saga my last build has been (a vintage race car, the Fiat 806 Corsa at 1/12 Italeri, see if interested the summarize here: Of course, many builds have ever been dedicated to the Tamiya P 51D Mustang, especially on Britmodeller... I saw particularly the Squibby one,...

www.britmodeller.com

Somewhere in there is a lengthy discussion about the back cushion. IiRC the conclusion was that there often was a cushion - not a parachute, not a flotation device, but a simple padded cushion.

 

[fubar57]

Thanks Andy. I got the above photo from there. That guy is way outta my league but I can get some good ideas from there

 

[Vic Balshaw]

Many thanks Andy, it's a gem of a site.