Quote Originally Posted by wylk View Post
I see a lockout product in a 1985 Master Lock catalog. Not much help, but it's another data point.
There is a brief article, "Safety Lockout/Tagout Products" in The National Locksmith®, January 2016. The president of a locksmith company is quoted therein as saying, we first began offering padlocks for Safety Lockout in the mid-2000s. The article also states that the business first opened in 1971!

Pete Schifferli