I had it explained this way:
You can buy original, authentic Porsche insignia, direct from Porsche or an authorized distributor, and put them on yourself. (They are damned expensive, I'll tell you, but if genuine, are in fact gold plated.) The commercial builders can and do supply these items but are not supposed to install them, although I guess we know how that works. They would just say that you (the owner) put the insignia on. I believe this restriction extends to the "PORSCHE" name only, as "Speedster" in any form or script is not a trade name, but a generic description of a form or style. Much like the word "Coupe" and "Spyder". There were (before Porsche came along) and have continued to be many other cars that were called "Speedster" or "Spyder". Likewise, I'd think just numbers, e.g., "1600", would be as generic as they come. But "Porsche" and the crest and such would all be subject to the proprietary interests of the company, if they would choose to have any. I have heard of no stories about anybody (private owners) getting called for this, but maybe some trouble in the past w/ commercial builders. Many after-market companies make all sorts of things w/ Porsche crest on their stuff, and I would suppose they pay a license fee or royalty or some such in order to do so. If you buy that insignia, then you pay your royalty fee in the purchase price, I guess.
The deal comes in hard at the point where a builder might advertise that he makes a Porsche this or that, so no words or pictures of their products w/ the private Porsche stuff is ever shown. Again, they can show "Speedster" because Porsche does not own that.
FWIW, and IMHO, as it was explaind to me (AIWETM -?-).