Plex Documentation → Your Media → Naming and Organizing Your TV Show Files
Plex is very picky with TV show naming and structure. Make sure you follow Plex guidelines.
Plex Dance the entire show after renaming/reorganizing.
The below naming & organization works on my system.
If you want to have them appear together, name the Lost Episodes as Specials for the 1968 original. TheTVDB lists them as Specials episodes 45-47.