FWIW, Whit Collins [the modern innovator of the 10 mm cartridge] had articles in the 3-71 and 12-72 Guns & Ammo. This is 1972.
"The primary tool of the G-Men, T-Men,
et al, has traditionally been the .38/357 bore revolver. The present arm of issue
is the Smith & Wesson Model 19 round
butt, 21/2-inch, loaded with Super Vel
ll0-grain hollow-point .38 Special ammo.
With the round butt's short barrel, .38 Spl.
loads lose only 125 fps from full .357s,
at the muzzle. Cost is cut considerably by specifying the .38.
Secret Service, and other federal agencies, are in the process of changing over to S&W's
stainless steel version of the Combat Magnum, the Model 66.
...Most agents working during my tour
were wearing the usual S&W Model 19
round butt, in Bucheimer-Clark holsters.
They had a choice of either an inside waistband
crossdraw, or a high-ride hip
scabbard on the shooting-hand side. Reloads
are carried in belt drops.
...AGENT NO.2
A younger man, this agent has his coat
buttoned over a Smith & Wesson Model
15 4-inch (Combat Masterpiece), which
is also commonly issued by the Secret
Service. ... He tells me later that he has a few split shot sewn into the lower lining of his coat, and that this makes it stay swept aside on the FBI-style draw. It's a trick he picked up from a canny old Scotland Yard security man who often accompanies Queen Elizabeth and the Royal Family."