If you look at that timetable, there's no way everything ran on the electrics; the 1717 overtakes the 1713 and is fast to Romford. Simarily the 1722 overtakes by Harold Wood. I only see 6tph in the "pure" all stations pattern that they have today (which gets 12+ tph now).
Actually it did. All these inner suburban trains departed on the Electric line, out through Stratford. Note how the one which is nonstop Stratford is the one which is after the 4 minute gap. Thereafter those skipping stops and overtaking were crossed to the fast line wherever there was the chance, with frequent crossovers, and then crossed back when they started serving stations again, generally the busiest ones. If they just couldn't get an adequate gap to cross over that evening, it just trailed the stopper, and arrived late. The omission of various inner stops was to spread the load, not to necessarily provide a faster journey. The "proper" fast line trains, most not shown here, ran on the Fast throughout.
It was all described in fascinating detail in a lengthy Modern Railways article in October 1964 "Peak Hour in East London" (which has been discussed here previously). Bear in mind this was done with local signalboxes, some of which still had manual points levers. The line had extraordinarily closely spaced searchlight signals, often little more than a trainlength apart, and two or three ahead were commonly visible.
And whether they switched some past Stratford to and from the main lines for a time, or not, it's the same service to the same stations that Elizabeth is now doing. But from just one London terminus.
Here's the magazine for anyone who wants to look it up :