By 'seatback' are you referring to the pylon behind the pilot's armor plate? If so, you'd need to use the bubble or 'blown' canopy version for Black 4 -- which was a shaped, almost tubular shroud instead of the 'strut' type brace used for the original flat-sided (low profile) canopy. Some good illustrations of the different versions on the "Aussie Fw190" page here. Scroll down a bit for the drawings.
For the 'Weekend' kit, it's probably one of the greyed-out 'do not use' parts in the instructions. I don't have reference to the part numbers, but it shouldn't be hard to spot.
Cheers