The opening is not square, and it shouldn’t be made square.
Mary Street faces the river. The south jamb has settled about eight millimetres over the height of the window, which is a little over a metre two. A sash cut dead square would bind at the top corner the first wet week.
The tenon is haunched, so the top of the stile doesn’t split when the rail shrinks.
I scribed the head to the existing lintel instead of packing it. The putty will be linseed, the glass is four millimetre, and the hinges are the ones that came off the old sash — cleaned, not replaced. New hinges would have needed new holes in a frame that has already had enough holes.
Come and see it before the oil goes on if you are the one who has to live with it. After the oil, I stop touching it.