The best reference I have so far is from 1970[0].

Waterfall is page 2. Reading past pg 2 you will see attributes of agile and various techniques described. The one thing that comes to mind is the blind men and the elephant[1].

Quoting from the wikipedia link (which quotes another):

O how they cling and wrangle, some who claim

For preacher and monk the honored name!

For, quarreling, each to his view they cling.

Such folk see only one side of a thing.

[0] http://www-scf.usc.edu/~csci201/lectures/Lecture11/royce1970... [1] https://en.wikipedia.org/wiki/Blind_men_and_an_elephant