Technically it has 9 settings if you count the half-number marks in addition to the actual 1-5 numbered marks.Some refer to these as prototypes but prototypes wouldn't be sold at retail with warranty cards and price inserts as these were. First year of the gold-plated "Gillette Executive" adjustable, aka "gold Fat Boy".The blade system adopted in 1930 is still in use today, and was used on razors as well after 1950. Other makers included serial or manufacturing codes, but we do not have the keys.Auto Strop and Rolls are two well known manufacturers who serial numbered their razors.
First known year of the "Gillette Adjustable" aka "Toggle".
Also first year of a plastic case for a Gillette razor: red/ clear styrene.