Pronunciation of final -o 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?
There is no contradiction; both of the statements are saying the same thing.


There is a final in Chinese that is pronounced [wɔ].
This is the final in syllables "luo", "ruo", "wo", "bo", etc.
It is sometimes spelled "uo" (as in "luo") and sometimes spelled "o" (as in "bo").
Specifically, it is always and only spelled as "o" following  b-, p-, m-, and f-.