Hello Chris,
I know you know a lot about buttons.... but I think Francois August Fonson made buttons around 1890 till start WWI When his son Jules came into the bussines (somewhere begin 1900, don't have a specifically year), the name was changed into Fonson & co . August was born in 1845, he died in 1920.
The Fonson bussiness started in 1848 by two brothers Jean-Barthélemy (1808-1882) and Jean-Francois Fonson ( no known dates), they used the mark Fonson Fréres.
It is not all clear for me,because there is not much information about those buttonmakers. I'm still looking for information.
During the lockdown I organised my buttons.... had a lot of time. In this way I put the buttons of the Belgian line infantry in a small frame. There were buttons of Fonson Fréres, A. Fonson and Fonson & Co. So these buttons were only made till start of WWI, later in 1915 Belgians got khaki uniforms with buttons model Belgian Lion.
Pic attached: I believe number 4 is French, still miss nr 5.