Marc-
I'm no Spitfire expert, so if anyone else out there has better info please share.
The seat back was padded, just like the texture on your seat. It was a black leather, with vertical ribbing just like yours. The seat itself was made from a material called bakelite, and was a reddish brown color. I have heard that they tried painting them in the beginning, but the paint did not stick well so they quit and left them their natural color. Here's a photo of the color:
Love the office on yours so far, you really have a knack for delicate painting and detailing!