Courser 40 in S/M is for a 14.5-18.5 inch torso. The M/L is for a 17.5-21 inch torso.
Bought one a couple of weeks ago. It is a nice pack. My only complaint is the top down duffle bag-like access to the main compartment. In other words, if you have an item at the bottom of the main compartment, you’ll need to dump the contents to get to it.