This is the most info I have found on it so far. It may have been issued to Gov't agencies (there are so many of them
) and not nessicarily the military.
"The Security Six line of revolvers was introduced by American company Sturm, Ruger & Co in 1971, as the new, most modern revolvers for police, military and civilian use. The line included three basic models - "Security Six" revolver with adjustable sights and square butt frame, "Police Service Six" revolvers (also known simply as "Service Six") with fixed sights and square butt frame, and "Speed Six" revolvers with with fixed sights and round butt frame. The first model was offered with three barrel lengths - 2,75" / 70mm, 4" / 102mm and 6" / 152mm, two latter models were offered only in two lengths - 2,75" / 70mm and 4" / 102mm. There also was fourth model in this line-up, the GS32-N, which was initially offered for government buyers; it was basically a slightly modified Speed Six revolver. Standard chamberings for the family were .357 Magnum or .38 Special, with 9x19 Luger / Parabellum added later (this featured a modified cylinder with patented extractor that had a spring ring which entered ejector grooves on rimless 9mm cartridge cases). For export to British commonwealth countries Ruger also made Speed Six revolvers in .380 British Service chambering. In 1975, Ruger introduced stainless-steel versions of all three basic models, which were produced concurrently with carbon steel models. Production of the whole line was ceased in 1988, with introduction of the second-generation Ruger GP-100 revolvers. Well over 1,5 million of Ruger's first-generation double action revolvers were produced. These revolvers were issued by US Border guard, National Immigration service, US armed forces, many police departments. Many revolvers were also exported to other countries.
In general, these first-generation Ruger revolvers were considered an excellent guns, strongly built, reliable and simple to maintain -all that with relatively affordable price. Many Service Six, Security Six and Speed Six revolvers are still sold in USA on commercial 2nd-hand market.
Ruger Security / Service / Speed Six revolvers are double-action guns with exposed hammers and swing-out cylinders. The trigger lockwork was assembled into the single unit along with detachable trigger guard, hammer was powered by durable coil spring. Revolvers have separate frame-mounted firing pin and automatic transfer-bar safety which permits the hammer to strike the firing pin only if trigger is fully depressed. Cylinder lock is operated by a push-button, located on the left side of the frame, behind the cylinder. Cylinder held 6 rounds of ammunition, and swung open to the left. Standard grips were checkered wood. "