These codes are found on the underside of the guard mechanism, or in cartridge razors, near the attachment mechanism for the blade.
However, blades made in those years were stamped with a date code: A for 1930, B for 1931, etc., plus 1-4 for the year quarter. In 1950 this system was expanded to include razors, too (see below).
If a vintage razor is found with its original blades, the blade codes can be used to establish a rough date for the razor.
Known specimen has a serial number of 2605 instead of a letter/number date code.
It also has a red dot on the neck near the adjuster ring and a red dot on the toggle switch. It was dated by the date code of 'B 1' on the included blades.
Dating Gillette Razors and Blades: "How old is this razor? Sometimes patent dates are a help, but they give you the front end of the date scale, and not always too exactly.