I just checked the Sears archive list, too....probably the same list Joe checked. According to that list, 158's were made up to 1980. I'm guessing it's early 80's.
I've got Kenmores (158 series), Singers, Whites, Nationals, & more BUT the Kenmores have got to be my favorites...with the possible exception of my Singer 201-2. Kenmores are really good machines.