Flange height refers to the hub, not the rim:



High flange hubs were popular for many years because they allowed for more tangential spoking, which was felt to make the wheel stronger. Low flange hubs became popular in the 1970s when Eddy Merckx started winning races using them. Low flange hubs allowed using longer spokes, which was felt to make the wheel more comfortable. And if Eddy could win using them, why not go for it?

Your early 70s vintage Frejus will likely have 120mm rear dropout spacing for a four or five cog freewheel. If your wheels are of a similar vintage, they should work without much trouble. If your wheel has a six or seven cog freewheel, you may need to spread the rear dropouts on the frame to install the wheel.