I had a set of those old Simplex plastic derailleurs on my mid 1960's Peugeot UO8. The rear worked pretty well, but the front was absolutely terrible. The front cage moves straight in and out horizontally and does not move down to properly guide the chain on and off the small inner chainring. As a result, I was constantly dropping the chain when shifting down to the small chainring and it took enormous effort to get it to shift back up to the large outer chainring. Eventually the plastic housing broke, and I replaced it with a Suntour that worked much better.