Sartel is a Belgian derailleur brand, based in Liège. I have seen at least two, and possibly three, Sartel derailleur designs. All were fairly straight-forward single pulley, pull-chain models.
My guess would be that Sartel was active at sometime between the late 1930s and the early 1950s.
One slightly interesting point to note is that I have seen a number of Sartel derailleurs on web sites based in Poland and the Czech Republic - and relatively few from web sites based in Western Europe.