I never heard that but I find question in that whole scenario. It doesn't matter if the gun sat for a while, residue doesn't create the bullet fingerprint of the gun, it's the barrel bullet combination. Even if residue or corrosion is formed in the barrel, the fingerprint of the bullet exiting the gun shouldn't really change.
Quoted verbatim from "On Target-Famous Assassinations and Attempts from Julius Caesar to John Lennon." By Stephen J. Spignesi, who must be quoting from Gerald Posner's book, "Case Closed."
What you say makes sense. I assumed the case must have been re-examined again in 1975?