Riders provided no voltage values for the input grid of the vertical oscillator. The other half of the 6sn7 does have the values given, and they rate normally (59 volts for the plate, -15 volts for the grid).
But, looks like we're getting closer to the problem, so I'll be digging into the grid circuit here shortly.
BTW, the Riders is now available at the ETF site:
http://www.earlytelevision.org/tv_sc..._diagrams.html
Glad to see many more schematics up at that site, maybe someday I won't need all my Rider's binders...