Frame year code, looks like this:
C for March, 9 for 1959
Boris, 10 is an even number!!! Sounds a bit high to me as well if it were an engine number, but production was getting quite high in the late 60s... If H-D are to be believed, 2000 XLH and 5000 XLCH made in 68... Total production around 27 000 in 68
Patrick