Ok, this is a trick question, isn't it? I looked in the parts manual and didn't find it (but it could always be hiding in a section I didn't expect like 'Transmission' or 'Exhaust'). All it is is a sheet metal piece that's not quite as wide as the 'T' of the frame and long enough to attach to the fiberglass floor in front of the front trunk. There's a hole in it to let water drain out and the left and right edges have a lip to stiffen it. Even if I could find a part number, I would be surprised if anyone stocked them. I would think they would expect you to make your own, but then in the next post someone will probably give you a part number and somebody who stocks them in aluminum!
Edit: New information
I did find it in the S2 parts manual (054A0201). Then I found that Spyder sells it as an "underlay body to chassis" them with their frame (probably available separately). Richard has them in his parts catalog (
http://www.parts.banks-europa.co.uk/orders_frame.htm). The interesting thing here is that the S2 and TC are two different parts so they could be more different than I thought! It seems everybody has them! Maybe it's worth a call to Ray at r.d.
