The Excavator Cog artifact can be found at East Garrison, in the southern part of Udûn.
Excavator Cog, Third Age, circa 2939
"This cog was spit from one of the great Uruk war machines, perhaps an excavator. As the Uruks push farther and farther into Mordor, they have torn up the land seeking resources to fuel their Dark Lord's massive war effort.
Excavators churn the ground to uncover metal for the forges, clay for the kilns, and any artifacts, weapons, or tools that might be of use."
"Here! Found it in Udûn.
What is it?
They got these machines, see? And they're digging up everything in Udûn. The thing tears the land up, then the slaves pick through everything. Clay and mud go to the kilns. If they find metal, that goes to the forges. Anything they can use they take back. And if they find an old weapon or piece of armor, they hand it off to the Uruk-hai, the big'uns. Maybe to study, maybe to wear, I don't know.
They're on the move for sure.
We must know what is happening in Mordor. We have other eyes there, but yours are sharp. You serve us well.
You will return there and report back. The council must not learn of your actions. Do not get caught."