Thanks shellymos, that's helpful.

I think I've found the answer with this quote:

"The Interpretive Manual of the SB5 (Roid, 2003) offers a table of Extended IQ scores for children who score above 150 IQ or below 40 IQ. Scores range as low as 10 and as high as 225 IQ. Based on Rasch scoring, the examinee is credited with all raw score points beyond the requirement to obtain the ceiling score of 19. "