Caminade's le Rectiligne system followed a tradition dating back to Edmund Hodgkinson's Gradient device, in that the 'derailleur' stays still and the sprocket cluster moves sideways to change gear. This provides a straight chainline at all time - hence the name Rectiligne (which you could crudely translate as 'straight').
This advertising leaflet, which I think dates from 1950, promotes a model called 'Le Rectiligne S-B'. I am unsure what the S-B means. The le Rectiligne system seems to being made available to many manufacturers (rather than only being fitted on Caminade's own bicycles), and S-B may refer to this in some way.