OK, so 乙 is the second of the Heavenly Stems (甲, 乙, 丙, 丁, …)
It’s often used to mean “second”, “two”, or “B”, as in “Part B” or Highway 7B.
So, here’s what I don’t get. Sometimes it means “one”. Often, especially in legal documentation, you will see … 乙份 or … 乙張. How did a word essentially meaning “two” end up meaning “one”? And why not simply use 一 (or to prevent the fraudulent addition of strokes, 壹)?
Perhaps originally it meant “one” and only later ended up meaning “second”.
Sometimes “A” can mean second in English. A house might be have the number 10, for example, and when a second house gets built on the same piece of land, it is known as 10A.
The fact that it has just one stroke and that it is pronounced yi3 suggests to me that the original meaning may have been 1. And according to Wictionary, it was pronounced “qyit” in Middle Chinese - c.f. Hokkien “chit”.
Not that that’s an answer to your question, but it might help you to look at the problem in a different way.
You’re right, it could be the other way around. It’s complicated by the fact that it looks like the Arabic numeral 2 (which itself originated as a ligature of two tally strokes).
And the idea that it may be cognate to 一 intrigues me. I need to consult the ABC Dictionary of Old Chinese when I next have the chance.
As I hinted before, I suspect that on legal documents they use 乙 instead of 一 because the latter can easily be changed into another number (e.g. 六 or a 十) by a fraudster. But it’s always strange to me to see it in less consequential writings like a high schooler’s resume.