Tony,I once made the frequency generator on a breadboard,and the 555 output was @ 42khz.I haven't checked to see if Stan used different caps and resistors with this one,but my guess would have been the same for both.
I don't see anything in that circuit that is used to adjustment for a max setting of frequency.The Gated Pulse Frequency Generator (fig 6) just adjust duty cycle,and the Variable Pulse Frequency Generator adjust frequency (fig 12).
So if you get 40khz out of the 555,then you get 4khz after the first decade counter,then 400hz,40hz.
Newguy,I don't think that the frequency should change after passing through the 74122.As stated above,it's only meant to adjust duty cycle.