Why did "2/10" mean "there are thieves here" in Hobo Sign symbolism?

It may have been a warning to keep your TWO eyes on the TEN fingers of the suspected thief.

Various British dictionaries of slang (e.g. Partridge’s Dictionary of Slang and Unconventional English) mention the phrase “two pound ten” (i.e. two pounds ten shillings of pre-decimal money) as shop staff’s slang to warn each other about a suspected shoplifter. The words would have been slurred to sound like “two (u)pon ten.”

Chart with several hobo codes:

https://blog.etsy.com/en/noted-the-secret-symbols-of-travelers-and-thieves/