The origins of ‘Number 2′/poo

I’ve never known where the euphemism of ‘Number 2’ meaning ‘a poo’ came from, but a theory came into my head the other day when I saw one of those toilets with a split-button flush – the ones with a small flush and a big flush. The small flush had one dot on it and the big flush had two dots. Could I have discovered the source of this fragment of our colloquial language? I hope someone knows…