The old 200-600 is based on the original 8.5-25cm (85-250mm) from the early 1960s. The barrels of the two lenses are pretty much the same except the 200-600 has a much longer rear section. The optics are also the same except the 85-250 has a convex element in the rear which effectively reduces the focal length by 2.4x (85-250 f/4-4.5 x 2.4 = 204-600 f/9.5-10.8 ... Nikon obviously rounded the numbers slightly to give 200-600 f/9.5-10.5)
