I've just undone the drain. I could be wrong but I swear the metal is definitely part of the tray - the hole is an absolute perfect fit, it's bonded both to the plastic upper and it sits flush into the board when looking from the underside. Will email a couple more photos.
Can you email a pic from above.
i think you are correct with the metal plate. It is fitted to some. Has the square fitting that is bonded to the top of the tray been moved?
i usually make the trap up with a good quantity Silicon too.