In 2005 Dong Shan 董珊 from the University of Beijing came across a dagger-axe that was held in a private collection.The axe, dated around 900-800 BC, contains an inscription:
一六一一一六，曰：鼑（鼎）止（趾）眞（顚）；鼑（鼎）黃耳，奠止（趾）。五六一一五八，X。
‘一六一一一六’ and 五六一一五八 are ‘numerical hexagrams’ that both convert to hexagram 50 ䷱. The text that follows it contains the name of hexagram 50 as well as phrases that are reminiscent of the line texts of hexagram 50.
Although the provenance of the axe is unknown Dong Shan does not think it is a fake, considering the patina and other features on the axe. This find would be the earliest object that directly links the numerical hexagrams to the Zhouyi, and it also contains the earliest text sample of it.
Dong Shan’s original article about the axe, published in 2011, can be read here http://www.gwz.fudan.edu.cn/Web/Show/2207. Pictures of the axe can be found at the end of the article.
Adam Schwartz writes in detail about the axe in his article ‘Between Numbers and Images: the Many Meanings of Trigram Li 離 in the Early Yijing’, published in the Bulletin of the Jao Tsung-I Academy of Sinology, Vol. 5, p. 47-84 (http://jas.hkbu.edu.hk/site/YCY/upload/mw_data/file/mw_data_2058_5b4411571ef04.pdf)