Is the light fixture used to illuminate the car number on the side nothing more than a license plate assembly? What kind is used?

It might be a license light, but not from a Porsche. It is a Hella part, and I have see some detailed pictures of it. It was only used for night events, so few cars were ever equipped with it. Same with the roof mounted light.

there are some photos of the light on Armando Serrano's Gt site

http://www.pbase.com/9146gt

i will try to attach a couple from that site re; your question

itsa a BMW 2002 license plate light

Yep, another shot of Scott's car. Photo credit to Lee

http://www.pelicanparts.com/cgi-bin/ppexpr...nse+Plate+Light