FYI for those unfamiliar, the Mil-Q-9858A standard requires fully interchangible parts, the usual test being to take ten revolvers, detail strip them, dump the parts onto a blanket. mix them together, and reassemble ten revolvers from parts selected at random, which must then pass all function, accuracy and environmental conditions tests meeting the NATO standard.
Ruger in USA and Manurhin in France (using Ruger furnished material, tech data package, and gages) were ever able to meet that standard. S&W and Colt revolvers have always been had fitted and could never achieve this.
After the last US government contracts were completed on the older Ruger "Six" design, the tools, fixtures and gages were sold to Manhurin as US production converted over entirely to the GP100 and SP101 series, revolvers being replaced in military and police service by autopistols. Sufficient spare parts were maintained to repair the older "Six" series revolvers for an estimated 20 years...
I have a 4-inch Police Service Six .38 Special 4" barrel which is my everyday carry around the farm, built as my armourer's school gun in 1985 and returned to the factory for refurbishment (aka FTR, or Factory Through Repair in Britspeak) in 2001 about the time the parts finally ran out.
The Rugers almost never wear out. Grand, sturdy guns...