When you instantiate an object in which the readonly property is required to be set, therein it will be set and subsequently, it cannot be changed.
Whereas the "required-but-not-readonly" property must be set in the object in which it is required, but subsequently it can be changed.
And it is this changeability that creates the problem by Qt not allowing the concept of "required and readonly" at the same time. It would be great if following is made possible:

required readonly property int itemNumber