This is the later of the two BSA derailleur patents that I have been able to find. It describes a derailleur that BSA released in 1949, following their distinctive, and rather elegant, tube design. This 1949 version has relatively small and simple chain guide plates and an integrated spring to provide chain tension. In 1950 this version was replaced by a very similar design, but which had larger and fancier chain guide plates and an external spring to provide chain tension that ran along the chainstay.
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