Spacecase: Tray: for 900 x 550 x 400 mm case
Model 550 with Tray and Rail kit Fitted
Rail Kit for tray fitted
SPC1105545 old model with tray (no need rail kit)
The L08 cases have a larger opening and the rails are needed for the tray to sit on, otherwise the trays would fall straight into the box.
The other boxes have a lip around the inside of the opening, and the trays sit on that, hence not needing the tray rail kits.