There are pressure springs that are keeping the focusing screen in place... well, the springs keep the glass on but not properly fixed. I compared the focusing screens of the new Yashica Mat EM and my older Yashicaflex and they seem alike. Glossy side up, matt side down (facing the mirror).
Luckily the seller has promised to take the camera back if I don't get it fixed. Calling to the repair service tomorrow.