Past Present and in Between in Pictures (Part 1)

^^^^ The first post-WW2 AEC Mammoth Majors introduced in 1946 were hybrid pre-war Mk.IIs / Mk.IIIs (Mk.II as you describe 7.7 engines and 2-stick transmission) but these hybrids had the 9.6 engine with the 2-stick set-up. By 1947 the ‘true’ Mk.III Mammoth Major was in being with the 5-speed gearbox. The 6-speed 'box was an export option from 1951, and a home market option from 1953. The 7.7 engine was used in the post-war build of 4x4 Matadors in 1953-56, and also in the Monarch Mk.III until it was replaced by the Mercury (AV470) in 1953/54. So the A.E. Evans tanker you saw at Barking was a pre-1939 Mk.II.