Hmmmm…. assorted things I'm finding (just from googling, sadly I don't have the Osprey books):
They would most likely fall under Troupes Coloniales, more specifically "Senagalese" Tirailleurs, a name given to all locally-recruited African troops no matter where they were from. They operated under French officers.
I'm not finding specifics on organization, but 1920 is right after WWI, so there are resources on how unit structure changed over the war. According to this, by 1918, a Company (under a Captain) was made up of 4 Sections (each under a Lieutenant and a Sergeant), which each had 3 Combat Groups (under a Sergent), each of which had 2 six-man Teams (under Corporals). These would basically be a automatic rifle (LMG) team and a grenadier team.
I'm assuming the Sergeants and Corporals could be African, the Lieutenants and Captain would be French.
(Although that's the organization for a main-line French unit geared out for cutting-edge trench warfare. Colonial organization could apparently be pretty different. Also, this is infantry as opposed to cavalry.)
They would be wearing dark khaki with yellow braid and fezes. According to this book, colonial forces typically ended up with outdated weapons, and some could be found using this 1874 single-shot bolt-action rifle in WWII. The best the average man would probably have would be these Lebel 8mm bolt-action rifles, which were standard issue at the start of WWI. Plus LMGs and SMGs and grenades and rifle grenades.
And maybe an armored car or light tank for fun, or some machine guns or artillery support.
Also, there's this wargaming post. They might know something if you dig around.
Edit: Further side thoughts. It's very likely that they'd be veterans of WWI, and being in the enforcement arm of the colonial hierarchy, the French officers would be corrupt as living hell, in bed with and serving the interests of white land owners and businessmen.