The first design of Huret Allvit, dating from 1958, exposed its complex and fragile mechanism both to corrosive road grit and to the vagaries of riders leaning the derailleur against walls etc... In addition to this, in engineering terms, it, unwisely, supported the main pivot pins of its parallelogram arms at only one their ends.
This improved, 1961, design, which stayed in use for at least two decades, has an outer 'shell' that both protects its guts and supports the pivot pins at both ends. These were two great steps forward. Despite this I still remember these later Huret Allvits as either hopelessly full of play, or corroded solid.
Half a cigar.