Wikipedia says... up until 1995 the Border Patrol issued it's patrol agents .357 Magnum revolvers as their duty firearms, S&W or Ruger large frame, six-shot revolvers. Agents could purchase weapons from the list of approved weapons for duty carry. This list included Glock 17 & 19 in 9mm, Sig-Sauer P220 in .45 ACP, the Colt Python .357 Magnum and the S&W 19/66 .357 Magnum.
Pretty sure Bill Jordan carried the .357 Combat Magnum after they came out in 1956. Before that my guess would be the .357 Magnum N frame. He started in 1941 I think. Here is a picture of my first year .357 Combat Magnum that shipped in August 1956.