I knew of Kuhnhausen years ago when I was in the business.
Due to so many people asking, I wrote the NRA Dope Bag editors and asked them for information.
They wrote back that Kuhnhausen had retired to Idaho and had died there in the late 1990's-early 2000's.
They said that he was a VERY private individual and preferred to limit his public exposure to the information he had on the back pages of his shop manuals.
From my personal knowledge, he ran a gun store in California?? and was a trainer of gunsmiths, especially for the gun companies.
That's one reason his manuals address gun repair by the factory methods of "doing it RIGHT", and none of the old time "get it to work SOMEHOW" heating and bending or parts making methods.
He was a factory authorized repair service for just about every gun company that sold in America, and was best known as a trade shop that provided top level gunsmithing to other gun shops.