The tie as evidence
Welcome, Jason. The tie is suspect for many reasons, as you have indicated. As such, it has limited "legal" value, if any. But it does carry significant evidentiary weight: DNA, metal particles, and spores. Also, its trail through the FBI's custody also tells us something about the FBI, particularly how it functioned in 1971. So there are many ancillary benefits to studying the tie.
What is the nature of your concern/interest in the legal standing of the tie?