A similar head badge decal was used on '83 Nishikis, and TMar has said that he's sure that Lamplight's gray Prestige is from '85 -- it has a different head badge design. It hardly seems to make sense that Nishiki redesigned the badge and then reverted later to an earlier design. Still, components that appear to be original are probably the best way to date these things from 20 years ago (because they were evolving quickly), unless there's a catalog or other evidence. Aero brake levers and sidepull brakes, combined with downtube-mounted shifters sort of put this bike in the mid-late 80s. Another tell-tale would be rear dropout spacing: 120 for 5-6 speed, 126 for 6-7, 130 for 8-speed and up. Sort of the beginning, middle and end of the 80s.

Check out the '83 catalog pages here (with a lot of other Nishiki photos in parent directories):
http://www.slackers.net/~skoop/nishiki/catalogs/1983/