WOW! Thats an expensive mistake. There is a brand new one on USA evilbay at the moment for buy it now $225 US. http://www.ebay.com/itm/Honda-NSR-250-MC28-factory-OEM-seat-new-in-the-box-Part-77100-KV3-950-/181715520262?hash=item2a4f16cb06&vxp=mtr
Maybe you can check the finish on that one?
There are a couple of possibilities as to why there is a difference. My guess is the company that supplies the seat to Honda may have changed over the years so its possible that the material used was slightly different. The HRC senario seems unlikely, firstly the part number sticker usually has a HRC label on it, and secondly the race bikes of that era generally (but not always) used a piece of foam stuck on a single piece duck tail.