Halliday & Matthiessen (2014: 295):
These are not, in fact, syntagmatically distinct from ‘identifying’ clauses; the clause the piano is Peter’s could be either ‘attributive’, ‘the piano is a member of the class of Peter’s possessions’ or ‘identifying’, ‘the piano is identified as belonging to Peter’. (Note that the reversed [form] Peter’s is the piano can only be ‘identifying’.)