More powerful JT9D engines (therefore more range) and an internal extension to the Upper Deck - it goes further back inside the 'bubble' (which has the same exterior shape). I think some early -200s (definitely not -200Bs) still came with the three window Upper Deck instead of the later, classic ten windows. Probably some avionics and other improvements. I know Qantas didn't want -200s, they waited for the -200B, so the difference must be more than just cosmetic. I think it was mainly in the engines.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz