I've farmed this for over upwards of 7-8 hours on and off but it doesn't seem to drop. I have Engineering to 450, I have all the materials required to construct it once I learn it, yes I am salvaging the kills. Often mobs will have no loot and salvaging will give no loot (about 35% of the time).
I would just like to know if this is a bug or I'm just insanely unlucky.
It should be noted that I've farmed this on retail (once during WoTLK and once during Cata -- Different characters) and it never took me more than an hour.