I'd think there would be an orderly process to produce discovery items. Trading copies of things casually at meetings called for other reasons just wouldn't happen, I don't think.Yes the FBI has a copy of everything given back to Laundries. This would stop Bertolino or anyone else from destroying it or altering it. (Seeing he refused to give Rielly a copy when they met).
Plus, as long as there's a MTD out there, no discovery would be produced now anyway (and I'm quite sure Reilly knew that. I'm sure too he would have done the same thing in SB's shoes no matter how he may be portraying SB's action in interviews. )
Finally SB isn't representing the L's on the case Reilly wants the letter for. It would be exceedingly unprofessional for him to turn over documents without the input of the lawyer representing the L's in the case. I'd think. So SB's "*refusal" makes perfect sense to me. (But I'm not an attorney.)