I have some info on Citizen Crystal 7 to share today. This old Crystal 7 ad above suggests that the name “Crystal 7” came from the use of BK-7 crystal. Unlike Seiko 5 which was derived from 5 features of the watch, Crystal 7’s name came from its crystal.
However, there were some features that were used as selling points of the watch. They were:
1. Thinnest automatic day/date watch
4. Strong and clear crystal
5. Easy adjustment of day/date