The identification of wonders 21-24 as the separate "Wonders of Mona" comes from the manuscripts K(2) and L (appears derived from K), both of which have the inserted title in red de mirabilibus monie insule* (Gill* notes this also of the "Royal MS" [? there are several]). The Irish versions have Inganta Manann Ann So Sis [Modern: Anseo Síos]* ["The Wonders of Manann Down Here" i.e. listed below(?)].
The contents list (c.1543 CE) in Manuscript C lists these wonders as De mirabilibus Monie insule, quo Anglice Engleseie vocatur, id est insula Anglorum* ["The Wonders of the Isle of Monie, which in English is called Engleseie, that is the Island of the Englishmen"] (the etymology is probably wrong). Strangely, Wonders 2 to 13 are also labelled De mirabilibus Monae insulae in Morris' Latin text*, however, this is in parentheses, and it is not clear where this label came from.
Initially these sites were identified as being on the Isle of Man (at least since Moore* and Mommsen* [Zimmer?*]). However Gill*, without any apparent knowledge of the contents list, identified the location as the Isle of Anglesey. His reason for this was largely the somewhat weak evidence that Wonder 24 could be the island's Maen Morddwyd, though he does invoke the more convincing Mene = Menai identity and note that the whirlpool Cereuus is probably Pwll Ceris in the Menai Straits. He notes Llwyd* had claimed the wonders for Anglesey a century before. The confusion arises because of the similarity of Latin names Mona: Anglesey (Ynys Môn in Welsh) and Monapia/Monaoeda Insula: Man (See, for various sources:*).
Wandering stones (Wonder 24) are not unusual, and raised beaches (Wonder 21) are more common on Man than Anglesey, however the names are strongly indicative of Anglesey: at best the inhabitants of Man might argue for our list being confusedly constructed from both a Man and Anglesey list prior to inclusion in the Historia, but the length of this section of the wonders makes this somewhat unlikely.