Rex was a budget brand that Super Champion introduced, possibly in 1938. It did not last long - possibly killed off by the advent of the Second World War in 1939. I believe that it was not revived after the War.
The Rex range included three models, the Rex Course (a fork derailleur with a downtube lever), the Rex Touriste (a fork derailleur with a toptube lever) and the Rex Professionnel (a fork derailleur with a chain detensioner). All three were Cadmium plated steel components (with no chrome plating or aluminium). They were also significantly cheaper than their Super Champion branded cousins. For example, in 1938, a Rex Course cost 65 Francs (as against 79 Francs for a steel, cadmium plated, Super Champion or 89 Francs for an aluminium Super Champion).The Rex Professional was a particular bargain at 85 Francs, compared with 120 Francs for the, admittedly aluminium, Super Champion Tour de France.
Despite the short life of the Rex brand, you do see examples even today.