I figured Brown had his information from Myers' book, because a google search for the file number of the memo lead me there as well. And - I might add - that was the only result that came up, which suggests there is no on line presence of the actual memo.
Belin can write to Myers whatever he wants but the fact remains that, in March 1964, the WC wasn't convinced by the available "overwhelming" evidence because they asked the FBI to investigate the dry-cleaner's label to establish a link between the jacket and Oswald.
Obviously the investigation ultimately was a dead end because, despite a massive search in the greater Dallas and New Orl?ans areas, the FBI could not locate the dry-cleaner who had attached that label to the jacket.
So, if they really had persuasive fiber evidence they did not use because they already had "overwhelming [witness] evidence to tie Oswald to the Tippit murder", then why in the world did they waste the FBI's time by asking for an investigation of the dry-cleaner's label.
Yet, they never fell back on the fibers after that... One can only wonder why.