The first time ever I travelled from Liverpool Street to Ilford, probably 1979, was in a Jazz 3-car AC emu, part of a full length formation with units with first class, likely for Southend. The Jazz units had a different, more open interior layout and different window arrangement. I assumed it was just normal for the AC stock to run mixed like this. Only more recently did I read it was decidedly unusual for them to run on the GE Main.
Few years later when I first came to London I lived near Wood Street, but apart from The City, whenever travelling into Central London, and particularly catching main line services there, it was far better to walk to Walthamstow Central and get the Victoria Line.
At that time the Jazz services were still run wholly by the 1960s 3-car AC units, 9-car formations in the peak, while the Shenfield service was run with 315s a few years old, which didn't do anything else. I never quite got how the Shenfield service was then so reduced that the Chingford/Enfield lines could be wholly changed over to "spare" units of this type, and subsequently they even ran out to Hertford etc.