Bruce L. Bergman wrote:

There are many grades, the US System is 'SO' for heavy duty rubber
jacket, Junior cord for light duty 'SJO'. The thermoplastic jacket
has a T in the mark - 'STO' and 'SJTO' The O is oil resistant, they
tack a W on the end for water resistant 'STOW'.

The J really means junior?

Wes