On a Facebook page, I saw and participated in a thread about someone selling a very bad clone, and had a Shelby tag with 5S306.
The car was originally a Detroit 6-cyl, and had so much wrong, it was actually hilarious. Yet the story was the seller was swearing the car was authentic! Many on the thread blew the car apart, and I also contributed, but kept the good secrets off the thread. The person looking to buy saw the truth, and did not buy as a Shelby, but may go back and offer badly modified Mustang money.
I cannot currently find the post, it may have been taken off, but wanted to put out the word about someone using the number.
Just an FYI.