Installing doors, identify this part.?

[foxhole]

View media item 96759Just about to install window panels to set of sliding bifold doors, included in kit is three corner packers with three metal plates, see photo, do the metal plates sit on top to allow movement of packer without friction, are these packers for top of glass when toe and heeling?

 

[foxhole]

Just had a look at top of door and there is an adjustable screw which will bear down on the packer, so plate must be to protect packer form splitting under pressure.Unfortunately two of the three screws are missing.