To correct you. The Duke of Windsor's actual names were Edward Albert Christian George Andrew Patrick David. Edward was his first name and David was is last middle name.

When William becomes King he will be William V in England and William II in Scotland. Remember we have only had William the Lion here in Scotland. The Scottish Post Boxes will probably have the cypher WIIR. They had ER for the late Queen.

Here is a list of Scottish Monarchs:

https://en.wikipedia.org/wiki/List_of_Scottish_monarchs

You will see the reginal number followed one another regardless of Royal Houses. For example - Malcolm II was of the House of Alpin and the next Malcolm to inherit the throne was Malcolm III of the House of Dunkeld.