And by "evidence of Oswald's guilt in the death of Tippit", Bill Brown means an identification by an "utter screwball" in an unfair lineup who had to be asked 6 times if she recognized anybody, shells not even found by law enforcement at the crime scene (which didn't match in number the brands of the slugs in Tippit) some with disappearing initials, and a gun that Gerald Hill pulled out of his pocket 2 hours after Oswald was arrested without any probable cause.
I don't need Helen Markham. Oswald is a cop-killer; no need to rely on Markham at all.
Shells were found by Barbara Davis and Virginia Davis and turned over to law enforcement. The chain of custody for these two shells is perfectly intact. These shells are evidence. They were linked, through ballistics, to the revolver taken from Oswald to the exclusion of any other weapon in the world. Whether you like it or not, these shells would be used to convict Oswald.
Unless, of course, you FINALLY have something to support your presumption that the shells aren't authentic. Do you?