I'm not positive, but along with the carb. difference FR mentioned, I think the Lightening had chrome tank and fenders where the Thunderbolt's were painted. In it's day the Lightening was considered a sport bike while the Thunderbolt was considered a tourer.
Lightning was a slightly quicker twin carb version. T'bolt had the chrome tank and large badges, chrome fenders as well. Internals were identical I think. You would have to step up to the spitfire for the fiberglass tank and mildly rodded motor. Hornet was next in line and a factory race bike - no lights etc.
why do you ask?