Maybe Z as in 'zed'?

It reminds me of yan tan tethera [0], leading to such glorious encodings as 'Yan-a-bub' (eleven, County Durham), 'Tetherabum' (eighteen, Swaledale and Tong in Yorkshire) and 'Fotherydik' (fourteen, West Country Dorset.) Each variant only seems to go up to twenty, but that's enough for hex...

[0]: https://en.wikipedia.org/wiki/Yan_tan_tethera