Got some information from a couple of members of a Flemish forum which specializes in bicycle racing history. They link the brand to Fons Mertens of Lier, Belgium. He and his brothers Jules, Paul and Albert were bicycle racers in the 1920's and 1930's.
Apparently Fons opened a bicycle shop in Lier, and also sponsored a team called "Mertens Sport" from 1946 to 1952 with local riders. I have been looking for pictures of the shop and of the team, but so far I haven't found any.
The Mertens family also ran a bar in Lier called "Sportwereld", in which the Lierse Bicycle Club was founded in 1932. LBC would become one of the most important racing clubs in Belgium, counting Rik Van Looy, Ward Sels and Herman Vanspringel among their members. The Mertens brothers were a driving force behind the club in the early years.
The 1959 club champions, with Rik Van Looy in the middle:
Van Looy would go on to become World Champion in 1960 and 1961.