"I would like to hear from other collectors of FBI proofs."
Based upon your web pages, https://jerryakatz.wordpress.com/#blog
; I trust you have already reached out to the usual collectors, dealers (including dealer data bases) and expertising folks for your inquiry. Also have you reviewed the various appropriate publications including the 50 year run of the Essay-Proof Journal which ended in 1993?
You may find more success with either an ad in The United States Specialist or have published there a quick article which includes the same request. It has an Essay-Proof Committee.