I can't comment on what the Frayse's may have imported and/or sold as "Libertas" in their shop, but this is (was) a well-known and established brand in Belgium.
Just to add to the database, here's a repetition of a few of the brands out of Belgium (some are "house brands" for frames built elsewhere:
Libertas
Superia
Flandria
Merckx
Main d'Or
Plum Vainquer
Unic Sport
Vagacini
Royal Nord Commander
Kessels