Double Keyhole Plates

I don't remember seeing anything about this before.

I noticed yesterday that on the left hand end of the side of some of the two car units there is a metal plate at the same level as the car number.

It is about three to four inches square and has two side by side keyhole shaped cutouts in it.

I presume it is to allow something ('Not To Be Moved' board?) to be clipped to the train.

I was trying to work out if it related to new windows but I noted both old and new windowed trains with and without plates yesterday.

Any further info?

Thanks

Andy