You'll find the plate for this one in one of the hidden places --

Hidden Places

-- in the area behind the wine bottles --

Behind the Bottles

-- when you FINALLY get the box under the dirt opened up.

How do I open the box under the dirt?

You'll get the sketch and a clue to the coffin after you get the third weird note.

Third Note: "A flower is a joy ... "

It's kind of hard to translate that sketch; it's clearly one of the middle coffins, but which one?

As it turns out, it's the one in niche 7, and here's one solution:

This coffin gives you a button with a number on it that will open the locked door in the cellar.

The Tunnel with the Locked Door