From classicahaving.com

What is the numbering system used on the blades? Numbered Blades

Some brands of razor blades stamp the blades so that if there is a quality control concern during the manufacturing process there is a way to identify the sides of the blade. Usually the blade has “1 2” on one side and “3 4” on the other. People frequently ask if the numbers are to designate a side that is better for shaving, but both sides are the same in that regard. Some men use the numbers for keeping track of the edge that they most recently used during their shave. The argument is that flipping the blade in the razor between shaves allows the edge to be worn down more evenly. However, most people just ignore the numbers and enjoy their shaves as usual.