The series includes titles by Ji Xianlin and Zhou Youguang; I picked up Numerals in Chinese (汉语的数目字) by Su Jinzhi (苏金智), of whom I know nothing except that one previous publication was a critique of Y.R. Chao’s scholarly work.

Here’s an interesting bit:

数目字在隐语中用其他文字来代替，这是文字的变异。商业活动中人们常常创造另外一套数目字，以达到经商赢利的目的。如明清玉器行流行的数目字隐语是，一为旦，二为竺，三为清，四为罢，五为语，六为交，七为皂，八为未，九为丸，十为章。十个数目字都隐含在隐语的文字中。苏州过去有一种数目字的隐语，也是采用这种字形变异的手法，只不过它直接说出变异的结果。这种隐语把一称为“旦底”，二称为“挖工”，三称为“横川”，四称为“侧目”，五称为“缺丑”，六称为“断大”，七称为“皂底”，八称为“公头”，九称为“未完（丸）”，十称为“田心”。

黑社会活动中，黑社会分子也经常有自己内部使用的一套数目字。

Numerals in coded argot frequently use character substitutions that take the form of written transformations. In commercial transactions, people often use a separate set of numerals in the pursuit of profit. A popular code for numerals in the Ming and Qing era jade sector ran 旦 (dàn), 竺 (zhú), 清 (qīng), 罢 (bà), 语 (yǔ), 交 (jiāo), 皂 (zào), 未 (wèi), 丸 (wán), 章 (zhāng), where each of the ten digits is hidden within a written character of the code. A coded argot once used in Suzhou employed the same technique, except that the result of the written transformation was stated explicitly: one was “the bottom of 旦”, two was “工 dug out,” three was “horizontal 川,” four was “目 on its side,” five was “incomplete 丑,” six was “broken 大,” seven was “the bottom of 皂,” eight was “the top of 公,” nine was “unfinished” [完 and 丸 are homophones, and an unfinished 丸 is 九], and ten was “the heart of 田.”

Gangsters frequently used their own set of numerals in their internal activities.

The book does not provide any examples of underworld usage.

I think I prefer the jade numerals as opposed to the hand-holding Suzhou system. Baidu Baike’s entry on 隐语, provides a slightly different version: 旦底、断工、横川、倒目、扭丑、交头、皂尾、分头、未丸. Ten is left off.