Quest Issue: a goblin penkeeper - Stone Key [Droga Pen Key 1]

[Titanx]

Does the stone key off the goblin penkeeper in Droga even drop/exist?

I only ask is because for the past three weeks I have been camping him, and while I know he is the rare spawn, and that is his rare drop, here is an estimate of what I've seen drop:

Killed him at least 60 times minimum, granted these are guesses as I wasn't keeping track of each drop I got.. but for three weeks straight, a few days with 10+ hour sessions logging in to kill him every 21 minutes, something seems off:

Bone Key - 8
Bronze Key - 25
Copper Key - 10
Iron Key - 10
Steel Key - 7
Stone Key - NONE

Thanks!