Indicating jeans that underwent the process of sanforization. Sanforization stabilizes the fabric before it is cut or washed, by stretching and pre-shrinking it. It reduces the chance on shrinkage to less than 3%.
The process was named after Sanford Lockwood Cluett and was patented in 1928 and first used in 1936 by J.C. Penney Big Mac. Lee jeans soon became sanforized, Blue Bell used them on their overalls and the Lady Levi’s introduced in 1938 were sanforized too.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary