Thanks to attentive people, I was made aware of another illegal copy of Periodical via Github (https://play.google.com/store/apps/details?id=com.droidfactory.periodtracker.menstrual.calendar).
In this copy all references to the original as well as the help texts have been removed. In addition, advertising was added that can be removed for a fee. As with all other copies (here, here, here, here and here), I reported the copyright violation to Google and also wrote to the publisher.
It took more than a month, but Google responded and will delete the app: