Once the rivet patented by Levi Strauss and Jacob Davis in 1873 expired in 1890, they decided to give work jeans numbers. Besides the cheaper 200 series, there was also a 500 series.
Number 501 is the only model that has survived. The design has undergone slight changes over the years. Nowadays, the 501 model is mainly known for its double arch stitches on the back pockets, the leather label with the two horses and the red tab. LS&CO initially used common work wear materials such as canvas and denim.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary