With Longines Extract from the Archives confirming production of the watch with stainless steel case and 12.68Z calibre and its subsequent sale on March 8, 1941.
The present example is a representative of the so-called "tre tacche" (Italian for three notches) family. It is defined by the three notches present on the screw back, which fit the case back opening tool. As a matter of fact, this is one of the first series of watches to adopt the screw back case construction as a countermeasure against dust, water and humidity. Originally marketed toward professionals and, especially, military personnel because of its improved resistance and durability, it soon became apparent that such features were held in high regard by everyone else as well, and the methodology became virtually the standard, with some notable exceptions such as the Patek Philippe Nautilus, for water-resistant wristwatches.