The oldest source for the coin method

During the last meeting of the Dutch Yijing group there was confusion about the assignment of the numbers 2 (yin) and 3 (yang) to the sides of Chinese coins. Old Chinese coins have four Chinese characters on one side and the other side is blank or has two Mongolian characters. When I looked for Chinese sources on this I found that there isn’t much agreement on the designation of the numbers, one of my books says that the side with Chinese characters is yang (see picture), and in this lecture Moxiang Liao 廖墨香 seems to follow the same designation, but there are websites that say otherwise. Curious about the origin of the coin method and wanting to know how the Chinese people in the early times did it I did some digging.

Bent Nielsen says in his book Yi jing Numerology and Cosmology:

Nielsen says that the earliest reference to the coin method is to be found in the commentary of Jia Gongyan 賈公彥 (who lived around 650) to the Yili 儀禮 (儀禮疏). Wanting to know how Jia referred to it I looked in my digital version of the Yili, which incorporates the commentary of Jia Gongyan. I found the following passage:

今則用錢。以三少為重錢， 重錢則九也。三多為交錢，交錢則六也。兩多一少為單錢，單錢則七也。兩少一多為拆錢，拆錢則八也。

In this passage Jia uses the words shao 少, ‘few’ and duo 多, ‘much’ to name the two sides of a coin, but it isn’t clear what is what: is shao the side with the four Chinese characters or is it the other way around? My dictionaries don’t say anything about this either. But when I looked for the word danqian 單錢, which appears in Jia’s passage, the  Hanyu Da Cidian 漢語大詞典 dictionary was helpful:

古代錢筮法術語。謂擲三錢 而成二面一背，象徵少陽之爻。
Technical term from a coin divination method from antiquity. When you throw three coins and you receive two mian 面 and one bei 背 it is called like this. This symbolises a young yang-line.

‘Two mian 面 and one bei 背’. What is mian and what is bei? In this case the dictionaries also assume that this is common knowledge. The Hanyu Da Cidian says that bei is the reverse side of a coin (錢幣反面的專稱), well, that doesn’t help much. Fortunately Jack Chiu helps me out with his book The Secret of Wen-Wang Gua. He says:

Chinese people used to call the side with Chinese characters the Face or Mian 面, and the other side the Back or Bei 背.

(p. 82)

Concluding: the side with four Chinese characters (mian 面) is called duo 多 (probably because this side has ‘many’ Chinese characters?), and the other side (bei 背) is called shao 少 (because this side has no or few Chinese characters). Knowing this we can translate the passage by Jia Gongyan as follows:

今則用錢。以三少為重錢，重錢則九也。三多為交錢，交錢則六也。兩多一少為單錢，單錢則七也。兩少一多為拆錢，拆錢則八 也。

Today coins are used. With 3 shao (= the blank side or with two Mongolian characters) one has chongqian, this is 9. With 3 duo (= the side with four Chinese characters) one has jiaoqian, this is 6. Two duo and one shao is danqian, this is 7. Two shao and one duo is chaiqian, this is 8.

If Nielsen is right, and the sources are correct, then this would be the oldest reference to the coin method.