Evan: I have always drilled my own (even though that admission is likely to bring out all of the listmembers who will remind me that I'm courting death or at least instant road rash by doing so). I don't recall ever seeing a stock stem that came so drilled or so cast by the manufacturer. Tho' of course I could be wrong..
(in advance to all of the "you're courting death" listmembers... yes, I know, but in very close to 30 years of lightweight cycling I've yet to break a stem or see a broken stem.. however I take care of my gear, do not overload critical parts.. and most importantly have never worked as a "wrench" in a LBS..)
Dennis "still riding (carefully) components from the 70's" Johnson in chilly forecast 28 deg. Visalia CA