Main types of boxes as standard 20' and 40' didn't change much since end 1960s.
I agree you about "since end 1960s".
In the other side written in US ARMY's CONEX page linked by max_thehitman, those early containers in the era of Korean and Vietnam were smaller than today's ISO containers.
About CONEX container is:
(Length x Height x Width)
2.6 m - 1.9 m - 2.1 m
8 ft 6 in - 6 ft 10 in - 6 ft 3 in
from here:
https://en.wikipedia.org/wiki/Intermodal_containerIn the section of "Small containers" tells us, US ARMY still uses smaller containers called Bicon, Tricon and Quadcon.
Bicon, Tricon and Quadcon are ISO 668's 1D / 1E / 1F.
Height x Width are common as each 2.438 m (8 ft).
Length are 2.991 m (9' 9.75") / 1.968 m (6' 5.5") / 1.460 m (4' 9.5").
from here:
https://en.wikipedia.org/wiki/ISO_668Not few photos show us today's US ARMY uses also 20ft ISO containers.