Speaking of rare brands, at least on this forum, here's one I acquired recently: a Visser Vainqueur.

It was the house brand of Teun Visser, who owned one of the bike shops in the Rotterdam area that catered to the local racing crowd. Visser even sponsored a small Cat 1 team in 1970, the "KRO Brandpunt Buitenspel Visser Vainqueur" team:



He did not build his own frames, but chose to outsource the frame building to reputable builders like Jacobus de Jong (the enigmatic "Witte Ko", who I'm told would rather go fishing than invest in his name as a frame builder) or buy them through Wout Verhoeven, who'd have them made in Belgium.

In this case the evidence points to Belgium, specifically to the shop that was started by Arthur Maertens shortly after the Great War. Arthur had been a talented racer (5th in the 1913 Tour of Flanders), who'd seen his career cut short by the war. He changed his name to Martens at some point in time, and after WWII his son Maurits took over the shop. They mostly built frames for others to put their names on. Both for the trade and for sponsors that needed high end machines for their pros. Michel Pollentier and Jempi Monseré, among others, have raced Martens-built frames.

The evidence constitutes of a panto'ed "D" in the lower head lug, which leads me to believe that this frame was originally built for Diamant, a Belgian brand, and also a known Martens customer:



How it got to Visser's shop I don't know, but given the pragmatism the Rotterdammers are known for, it's not very surprising either. Someone needed a decent bike and was in a hurry, would be my guess.

Anyway, here's the bike as I've just rebuilt it for Eroica Limburg, hence the 28mm Paselas. I need to make more miles on it for a more definite verdict, but so far I've found the 531 frame to be very responsive as well as very comfortable. So much so, that I spent the first ten kilometers of the Eroica ride worrying that I'd somehow misread the pressure gauge on my track pump and didn't have enough air in my rear tire.