Yes Stephen as you said pre 90 models had a simmilar design whith mc16-mc18 but they had problems with temperatures and after they made this design with displaced cylinder studs to prevent more coolant.
http://img152.imageshack.us/img152/4600/nsr500v436.jpg
I have also a video with some minutes inside the 1988 HRC production erea with lots of terra (NSR250-NSR500)beauties inside,take a look after 1:14 http://www.youtube.com/watch?v=ldTwmeqEqVQ&feature=relmfu