Are Thurg armor drops only on the mobs listed, or other mobs in Kael?
 

I'm killing the Watchman around the bank and commoners area, mobs like Bexlend and Tylem, because I was told the mobs at 9 and 10 on the map can drop eroded leather, which is what I'm after. But I've seen nothing yet.

It's very random and rare. Almost better off farming platinum and buying sky shrine gear

The lowest level mobs in Kael that have a possibly (that I know of) of dropping Thurg no drop quest items are the "Lieutenants" in the 4 way area (I.E. Lieutenant Vellos, Coldforge, Peldiyrem, Stormeye, Frostbite). Other than than drops are random from Arena and King Tormax area.

You will notice them as being the Kromzek storm giants in the blue armor (no long beard). I will link you the P99 wiki entry and you can check the map locations for specific spawn points. Drop % is close to 2% from the Strom Giant Lieutenants.
https://wiki.project1999.com/Lieutenant_Vellos

This is a reference as I never seen armor drops from lower mobs than these.

If you like I did a video on this area to provide a frame of reference. If you want to take a look PM me or discord me and I can send you the link.
Sarleon#8503

They don’t call them lootenants for nothing , hope that helps.

You make good youtube videos man.

Why thank you! It is my pleasure.