According to Wiedenhof's A Grammar of Mandarin, page 43,
The final spelled as -o is only combined with the initials b-, p-, m-, f-. This vowel matches the vowel part of the final -uo [wɔ].
However in page 45, the author states
The fnal -uo [wɔ] is spelled as -o before the labial initials, b-, p-, m-, f-.
Can somebody clarify the apparent contradiction?