I've come across a fair number of vintage bikes with Simplex components, but the front derailleur would be made by a different brand. I read that the original Simplex front derailleurs used a plastic clamp that was prone to breaking, and people would have to replace them when they inevitably broke. My brother bought an all original 1970s Peugeot from a thrift store with one of these mythical Simplex derailleurs.
Sure enough, there's a crack right in the weakest part of the clamp:
My guess is that these front derailleurs all failed within the first 100 miles or so, and got replaced with whatever metal derailleur that the bike shop had in stock.