The models that may have been used in these USPOD holsters are S&W Model 36 with a 2-inch barrel. If anyone is interested in handgun usage on Railway Post Offices (RPOs) or a 1956 video of how mail was sorted in trains, please visit www railwaymailservicelibrary org (add dots. Also, there is an article titled "Gun Play at the Mail Car" by Don Rohrer that is available free as an Adobe Acrobat file.