I am on very thin ice here, but the "Arduino Cookbook" by Michael Margolis includes some sub chapters in chapter 18 about changing the PWM frequency. I just bought the Kindle Edition, and combined with my gigantic lack of knowledge regarding the Arduino I don't know if this make any sense but:
In the mentioned chapters he use the term "prescale factor", accoding to the book TCCR1B ( timer 1) have a table showing the following columns, TCCR1B Prescale Value, Prescale factor (divider), Frequency:
1 1 312500
2 8 3906.25
3 64 488.28125
4 256 122.070......
5 1024 30.5175.....
The sketch in the example use the Serial Monitor so you can type in a digit from 1 to 7 to choose the frequency. The TCCR1B is the only register mentioned for timer 1.
And that is all I understand, sorry if I only added to the confusion!