Hard cover for hip ring???

[Grant Bowker]

The attached photo was seen in the thread "in the background". It seems to show a hard cover over the hip ring in the roof of the Cab 13 with the cover being hinged to open toward the centre of the cab. It also seems to have stiffening ribs in a cross pattern. Does anyone have any idea whether this was a one-off, a unit level modification or a production option (or a figment of my imagination)? If it was a frequent occurence it would help keep the interior dry, much better than the canvas cover that I thought was the standard.