Here is what the letter contained:
MILADY, Be at the first ball at which the Duke of Buckingham shall be present. He will wear on his doublet twelve diamond studs; get as near to him as you can, and cut off two.
As soon as these studs shall be in your possession, inform me.
Chapter 15
On the day after these events had taken place, Athos not having reappeared, M. de Treville was informed by d'Artagnan and Porthos of the circumstance. As to Aramis, he had asked for leave of absence for five days, and was gone, it was said, to Rouen on family business.