Could be a special order. To my (limited) knowledge the brown interior was reserved to the 2004's Special Edition, also called 50th Anniversary or 550 Spyder. Could be really wrong here and customers could made it an option too, dunno.
50th Anniversary:
https://s3-prod.autoweek.com/s3fs-public/styles/width_792/public/405100708-2.jpg
ps. after re-reading your post and running google image on boxster "natural brown" interior I'm getting all kind of colors ranging from lighter to darker beige. So again, very wrong I could be here lol