Burton catalogs first mention of quadratic sidecuts was in the 1990 season for production boards. Dimensions of the 1989 Safari Comp III were 295mm nose, 266mm waist, 293mm tail, 147cm running length. Using the old approximate formula Radius = Running Length2 / (8 x Sidecut Depth) gives a radius of about 19.3m. Safari Comp II was about 16m, and it felt like it.
But this thread is kind of silly, this board belongs on a wall.