Im afraid it is not going to stay closed without the nub. Personally (yes, Im sorry for my unsolicited opinion) I feel strongly that you should return this to the sender. The bag is not "perfect" as described if this nub is broken off. Looking at those pix, that's a pretty big gap! Even if you managed to lessen the gap, I dont think it would stay closed well, or tightly. It would probably constantly pop open. If Deigo says they won't fix it because it is too old, I don't think you are ever going to be truly happy with it....
Just my $0.02...
You WILL find another even more perfect black stam.
Sorry~