Technically you are probably right about state law, I don’t know. However, under federal law (18 U.S.C. § 922(k)), if he had crossed a state line with altered or missing numbers, it would have been pretty serious. That became law in 1968.
How experienced do you have to be to just put the murder weapon in the evidence room with a tag and leave it there?
I don’t think it was a grand conspiracy, but perhaps someone wanted it to be quashed after they got wind of things. I’ve seen that happen, investigations are dropped without a word. Evidence goes missing, crippling a case. Everyone in the world did not have to be in on it. LE is not some monolithic entity.
Anyway, the killer will never be known with certainty, the evidence had too much of a minor league or willful mishandling. But the DNA is in the system, and we will know who they are, within a year I would think.