If it were a reflection, i would expect to see other objects from within the cabin though
There does seem to be a dark shape around the object (window?) so its possible i guess.
But looking at this larger shot it doesnt strike me as a reflection
There is reflection from inside the cockpit, but it is that "dark shape" that you speak of at least in my estimation. The windows of the Grumman are square and that would make sense from my point of view.
I don't know what it is, but it sure if fun to speculate.