Mortise and tenon flat
Mortise and tenon flats are usually covered with canvas for painting, or lining material and black serge if they are intended to be masking flats. In this style of construction the corner joints are mortise and tenons, as are the joints attaching the shoe to the toggle rail. The diagonal braces however are screwed through the brace at angle to the frame.
In this close up of the toggle rail and shoe the pegs (shoe laces) can just be made out in the centre of the shoe.