Levi’s denim from before 1983 had a redline selvedge. Cone Mills, which manufactured the Levi’s denim fabric, added this red line to their selvedge to make their fabric stand out. At the time it was considered as good as a standard of excellence. Denim aficionados still argue that the red lines denote top grade denim. See also ‘Cone Mills’.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary