Done. Thanks for this Notification.
Best to download the version which is dated 11-6-2016 (11-June-2016). For installation of Plone 4.3 you simply delete the registry.xml file. See the two readme...txt files inside the package.
It took me quite a while to figure out why the booking of a periodicity was not working using the Edit button (The default Periodicity button worked already).
One single "_0" which Plone 4 added in a TAL instruction, which TAL on Plone 5 did not do was pulling my hair out for several days.... Now that beast is tackled and all functions work now smoothly.
Note that in Plone 5 the CSRF feature must be switched off as that breaks the functions.... I do not know how to solve that. Especially within the Javascript pop-ups.
Maybe somebody is willing to spend some research time on that item?
Of course a rewrite to Dexterity Types would be best, must this is pretty complex stuff.
I just did the pull request. As I am new to this I hope I did this the right way. Thanks for your guidance. Let me know if something went wrong and then I will (re-)try to correct it.