Rolex OP 116000 Clasp Question

[SCutshall]

I am a Rolex Rookie (just purchased my first piece, an Oyster Perpetual) and while staring at it (endlessly) was curious as to the numbers and letters on the tongue(?) or tang (?) -don't actually know what that part of the clasp is actually called... the part that folds in nearest the inside of the Oysterclasp?
The numbers read either "Y1S" or perhaps it is "Y15" (I can't tell for sure because I don't own a loupe and my bifocals apparently aren't as strong as I thought they were).
Any ideas? I Googled it but I can't find anything. Is it a part number or code for the entire clasp or what?

Thank you for reading!