for pbp3 Define OSC 24 is a valid define
from the book
2.3.1 DEFINE OSC
There are a limited number of valid numbers that can be used: 3 (3.58MHz), 4, 8,
10, 12, 16, 20, 24, 25, 32, 33, 40, 48, 64. These are the only frequencies for which
PBP is able to accurately calibrate its timing. If you use a system clock that runs at
a frequency that isn't listed here, your timing will be scaled when the program
executes.
24 is in the list

osc setting
Code:
                                                   USBDIV_2_1L
                                    / =========== 96/2 = 48 ----   usb
20 mhz  /5  = 4 :-  pll =  96mhz  :
      PLLDIV_5_1L                   \============  96/4 = 24 ---   fosc
                                                   CPUDIV_OSC3_PLL4_1L