I'm going to have to go out on a limb, and say that the information I can find about the SMPTE Slave Driver indicates that External Sync refers to a wordclock (1X)/ superclock (256X) switchable input. To the best of my knowledge, there is no such thing as a 256X blackburst generator.
Please take this with a grain of salt...
Less is not more. More is more.
Grace, Peace, V, and Hz,