NOTE TO Franz: All of this information here in the 'Main Quest' lore pages is not known by the NPCs unless they are specifically mentioned to know about a piece of information.
@P. Schuyler & Sons is at 44 Devonshire Way in the city of @Tarant. Inside you find @James Kingsford who fobs you off.
To continue, you can:
Kill him, quick and messy
Pick pocket his key
Convince him that you belong to the @Tarant City Authorities and he needs to cooperate (which can lead to his death)
You can then use it to go down the trapdoor in the back room of @P. Schuyler & Sons. This leads to a room full of zombies and some random treasure. Once the zombies are dispatched, you can go down the next trapdoor. This leads to a tunnel with even more zombies and traps. Go down the next trapdoor. Here you will find a dwarven tomb that is underneath @P. Schuyler & Sons.
You will find the Schuyler's Brothers ( @Winston Schuyler , @Niles Schuyler & @Edward Schuyler) here. To get the information you require you can
Kill them all, find they key and open the wood chest
Use your pick pocket skill to get the key
Come to an agreement with them to get the information (this will annoy @Magnus Shale Fist is he is in your party.)
Use you pick locks skill to open the wood chest.
In the end, you find that the ring belongs to @Gilbert Bates, so you must talk to him.