Cantilevers go way back further than the 50's. I've only ever seen one set as early as the mid 1920's, however the basic difference is that many of these did not use independent bosses brazed to the forks, but used pivot brackets clamped to 'D' section fork legs or from plates hung from the bolt hole in the fork crown. Philips and BSA used them in various disguises through till the 1950's, and all seem to suffer from rust. Perhaps the best known though and arguably the best functioning was "The Cantilever" by The Resilion Co. - from a design orignally by Raphael Roblin (Fr) and Vernon Blake (Eng.Nat) both living in France in 1926....... and improved on by the Resilion Co. in the 30's......
Sadly missed apparently when they disappeared in the 50's, despite such innovations as side-to-side adjustment/compensation. Resilion failed to notice post-war that the world was passing them by and 'improvements' such as locking levers (a desperate idea anyway) and aluminium brake levers were no competition for the lightweight products of one Gerald Nutland Burgess.
Bob Reid
Stonehaven
Scotland