The structure marker of is analysed as a constituent of the embedded prepositional phrase serving as Postmodifier in the logical structure of a nominal group:
Halliday & Matthiessen (2014: 425):
We noted above that prepositional phrases serve either as Adjunct or as Postmodifier. The exception is prepositional phrases with of, which normally occur only as Postmodifier; the reason is that they are not typical prepositional phrases, because in most of its contexts of use of is functioning not as minor Process/Predicator but rather as a structure marker in the nominal group (cf. to as a structure marker in the verbal group).
