Elmer Keith's FBI aquaintance carried a Colt SAA in .38-40 . Keith reported that at the time FBI practice was Agents could carry any revolver at least .38Spl . Maybeso , or perhaps that Agent simply refered to his sidearm as "Colt .38" as far as his employer was concerned.
In his autobiography , G. Gordon Liddy reports as a child he had strong memories of his FBI SA Uncle's issued .38Super . In his FBI days , he was issued an Official Police , but very quickly replaced it with 3.5in M27 .
An aquantance of mine was Vietman Vetran USMC , and by then former FBI SA .He worked Inserstate Fugitives , and was on the Field Ofc Swat team. His duty gun was a personal M19 , that had been tuned by Quantico Armorer. I had the privledge of shooting this gun , and indeed it was smooth. However by this time thru either a weak trigger return spring , stacked tolerences , or probably both , it would occasionally lockup. At the time it was carried in a generic knockoff pancake then issued by our mutual employer. Unknown which holster used during FBI. He was a curious mixture of gun person/ non-gun person. He had amazing shooting skills , but his entire knowledege of all things gun or shooting related came from Quantico. Either circa 1967 with USMC Ofc training , or circa '72 w/ FBI . Anything different or subsequent didn't exist, even subsequent FBI tought techniques was dismissed out if hand.
For a long time I was confused over references to Hank Sloan holsters , because I knew another Federal Agent of the same name , and at least partly overlapping years of service.
The father of a close friend , WWII winner of Silver Star, Dallas PD before becoming a Treasury SA . Retired as SAC , and in retirement years was pt Dispatcher for his Sheriff in West Texas . I don't know about early career , but from mid '60s to retirement carried Chiefs Special , with Treasury Loads. I also recalled they had monthly qualifications. Don't recall holster.