Clock frequency is formed by having FSB and multiplier. In you case FSB is 133 and multiplier is 7,5 so 7,5 x 133 = ~1000. The FSB is set by the motherboard so if you plug in 1.42GHz processor it will not run full speed, multiplier is in this case adjusted from the G4 processor daughter card or whatever it is called.
In PC world with early Athlons/Durons it was a common modding to unlock the multiplier and then adjust it and FSB to achieve higher speeds.
I have performed overclocking twice on G4's, first I removed the resistor on Dual 867Mhz to achieve 1100Mhz. Second one was adjusting multiplier up one on 733Mhz Digital audio to achieve 867Mhz.
Both of these had no real noticeable performance advantage, on Digital Audio I was of course able to install Leopard without permorming any addiotional tricks