Representing ancient Greek numerals in LaTeX turns out to be a bit more difficult than anticipated.

With regards to the history, I found this to be valuable and interesting:

https://web.archive.org/web/20151030220136/http://www.tlg.uci.edu/~opoudjis/unicode/numerals.html

For the digamma at 6, \varsigma is often abused. But \varsigma does not have a distinct character in the Euler font (it renders as \sigma); fortunately that can be rectified in text mode using \fontencoding{LGR}{\char99}.

The archaic koppa ("qoppa") is \char19 in LGR.