Mafac levers: The all-open version was early, used often with the brakes of other makers - for example, Herse used these on their bikes with Herse brakes. Mafac at the time, I believe, only made their cantilever brakes. I have seen them on bikes from approx. 1950 onwards. Before, most people used Lefol or LAM levers.
The half-closed model, still with dots between the letters, was shown in Le Cycle in 1952 as a new item together with the announcement of the Racer centerpulls (marked "Forge Dural" then, but already officially called Racer).
LAM also made brakes and levers. 1940s sounds about right. Most French brake makers went by the wayside or downmarket to cheap bikes, once the Mafac Racers were introduced. So you don't see LAM, Lefol, etc. on top-class bikes after 1955 or so. (It took a few years for the Racers to be adopted by the cyclotourists. Originally, they were intended for Racers - as the name implies. I think the Racers swept the racing world - except Italy - as soon as they were introduced.)
I don't know much about racing bikes, so the above refers mostly to cyclotouring bikes.
Jan Heine, Seattle