First Gen, if you will was 1987 only, first year of the TW. It had a separate lighting coil, the lights only came on if the bike was running. It had no front running light feature and the safeties were wired slightly differently from subsequent runs. It used a 1987-specific CDI unit. The left engine sidecover only accomodates the 1987 charging/lighhting system.
Second Gen was '88-'01. It had an improved single phase, self-contained charging system, CDI was specific to those years, 35 watt headlamp, had front running lights and essentially the same system for the entire run. Left side engine cover again only accomodates components from these years
Third Gen '01 to present. They have a higher output three phase charging system, higher wattage headlamp, their own corresponding CDI unit and self'cancelling turn signals. Nuther slightly different left side cover.