While Britain issued many sets of soccer cards prior to 1920, the explosion of international interest begins after the 1924 Olympics. South American issues and Continental European issues begin to increase.
So the key dates are from 1924 to 1938 - until the advent of the Second World War.
Note: there is a noticeable drop of any issues surrounding the First World War (1914-1918). Production begins in earnest from the UK by 1920 again.
I suspect there are many issues still undiscovered around the world
