Halliday & Matthiessen (2014: 394):
The word of is the generalised marker of a structural relationship between nominals. All these instances can be related to the different senses of of occurring in a Qualifier; e.g. (aggregate) the House of Lords, (portion) both Houses of Parliament, (quantum) a house full of treasure, (variety) a house of respite, (facet) the House of Windsor, (make-up) a house of three storeys. In all such examples, house is both Head and Thing. In measure/type expressions, on the other hand, the Head word has become partially grammaticalised; hence it is often phonologically weak (non-salient), and there is often indeterminacy about the location of other elements in the nominal group, such as plural markers and Epithets.