Which companies used Simplex dropouts during the 70s? Wow, that would be an extremely long list. For starters, I would imagine every single manufacturer of bicycles in France would qualify. I've seen them also on Dutch, Belgian, Spanish, Swiss, even Italian as well as on many Raleighs models... So, probably thousands of individual bicycle models. A MUCH longer list than models which had used Campagnolo dropouts.

Pretty much every company in Europe. It is rather like asking which models during the 70s had used one form or another of Reynolds tubing. Seriously, Simplex would probably qualify as the single most popular dropouts used throughout Europe during the 70s. And even the dropout models with integrated derailleur hangers were available with either threaded or non-threaded (the latter mainly, but not exclusively, found on French bikes).