As a now-retired Postal Inspector who was a Division Firearms Coordinator for 14 yrs, I can hopefully clarify a few points.
In 1986, the U.S. Postal Inspection Service contracted with Ruger to build a stainless steel .357 magnum revolver with a 3 inch barrel exclusively for use by Postal Inspectors. Although the Post Office Department had a long history of firearm purchases over the past 200+ years, ranging from shotguns & revolvers to submachine guns, this Ruger “Speed Six” Model GS33-PS (Government Service - Postal Service) represented the first (and last) time a handgun was manufactured to specific U.S. Post Office specifications.
After Ruger made delivery in 1987, this weapon was assigned to all Postal Inspectors and Postal Police Officers. Approximately 3000 were manufactured, all bearing a serial number prefix of 162 followed by a 5-digit number up to 23446. There are no special markings on the weapon such as a badge or the letters USPIS or USPS. However, included in the Ruger box with each of these revolvers was an owner's manual that showed the Postal Inspector badge on the cover. On the inside page was a letter from the Chief Postal Inspector (dated 3/15/1987), identifying it as the offical weapon of the U.S. Postal Inspection Service.
Approx 5 yrs later these revolvers were replaced with a semi auto pistol. Inspectors were then allowed to purchase 1 of these Rugers for personal use. As a result, some have since been re-sold to the public.