if you use 24MHz, you will find the center of the downlink passband on 10489.750-(24*390)= 1129.750 .
If you use 25MHz, you will find the center of the downlink passband on 10489.750-(25*390)= 739.750.
The 24MHz reference lets you receive the DATV transponder on a normal unmodified Sat-RX ; you can receive the beacon and some other individual signals.
The 25MHz reference does not allow that, because sat-RXes do not cover 740MHz.
If you want to receive with a normal 432MHz rx , you use a reference in the range of 25,78xxxMHz
Some even receive directly on 2m band.
If you use a rtl-stick or other sdr device for rx, it is not important, what reference you use, as long as the device can receive the resulting frequency.
Identify the built-in crystal by checking the LO frequency printed on the plastic cover, 9750 means it is 25MHz.
25*390=9750 , but it will also work with 24.xxx or 25.78xxx MHz.
Edit: The description for the lnb from the link you provided says 9.75GHz. Voila'