Not sure what the 1 stands for. I have looked at several helmets and the letter at the top has been M, S or L. The numbers were different in many but 1 was the most common. Could be a lot number, mold number, I don't know.
I did some checking and no one seems to know what the numbers are. 1-6 are known. Mold number or manufacturer are the most common speculation but the manufacturer is also stamped on most of them so that seems unlikely. That leaves mold number.