Don't let anyone fool you into thinking this is any easier to obtain than the Fisher's Torque. It is indeed the brown caskets from Maze of Shakrami off any mobs (even the low level ones) with about the same rate of luck.
It can be argued whether this is worth getting or not, but for singling out GP items it can absolutely have situational fishing uses. Good luck!
The new fishing system hints at what type of fish is on your line depending on your skill. Discernment is one's ability to judge. "Angler's Discernment" is probably the name of that trait.
Just obtained off TW mobs in Maze of Shakhrami. Inside a 6 try chest with another item, potion of some sort. took 5 days of ~8 hrs/day to obtain. 1/148 on successfully opened brown chests. Happy hunting.