Filter Drawer/Window?

One of the specs for the new DA* 300mm F4 is a filter drawer or window (depending on which site you look at) does anyone know what this is?
The size of the filter is 77mm, would this be a threaded filter?

It probably refers to placing gelatin filters somewhere at the rear of the lens, in addition to the normal front-rim screw in filters.

slot in lenshood for rotating a polarizer?