check out this 1940s racing Vittoria Margherita! chain tensioner, derailer, and whatever these are. I posted stills, but see the video past the link

This system has rod-controlled arms on the chainstay which are moving the chain when the rider backpedals. It is simple and easy and certainly much better then the alternative of getting off the bike then removing, flipping and replacing the rear wheel. The system is reliable and the tension wheel leaves just enough ground clearance, an important consideration in an era of bad roads. Unlike more fragile gear changing systems which were on the market in the 1930s and 1940s, the Vittoria still worked when clogged with the clay of the road.

http://www.disraeligears.co.uk/Site/Vittoria_derailleurs.html
https://www.facebook.com/diaspedalando/videos/314100809298868
https://www.steel-vintage.com/taurea-vittoria-margherita-1940s-road-bike-detail
http://www.disraeligears.co.uk/Site/Cambio_Vittoria_Margherita_-_leaflet_scan_3.html