Well it took awhile, but I finally got an original HP 8698B RF plug in manual for a reasonable price.
Those clowns on that auction site are pretty proud of their manuals.
In any case, the 8690B sweep oscillator manual doesn't disclose the various disabled functions when certain plug-ins are used, i.e., the "standby/RF" switching in the oscillator doesn't apply with the 8698B plug in. Good to know because I was worried that there was something that needed to be fixed.