[stephen f. polyak]

Speaking with my aeronautical engineering hat on . . . Practical considerations would make a slot unlikely. Such an opening would allow weather in (rain) and would likely admit/produce noise. I suggest it’s the means by which the top canopy panel is slid open/closed for ventilation from within the cockpit at the pilot's choosing. It appears to be a bonded piece of Plexiglas to reduce visual obstruction. The placement and shape of the part allows its use with either hand, or possibly even with a nudge of the head.