FWIW, based on my own experimentation the clock base is 5 MHz which corresponds to neither variant of external oscillator. (I decreased the DSCOUNT value by 9000 and saw a timing difference in the DS0 wakeup sequence of 1.8 ms -> 5 MHz.)
Update: measuring more exactly, it seems the 5 MHz is actually closer to 4.8 MHz which of course is 1/4th of my 19.2 MHz.