The 55-58mm normal lenses were made when the SLRs first came out, and were competing with rangefinder cameras. The selling point was that not only did the viewfinder show what the lens sees, the image was also life-size, just like with a rangefinder.
The 50mm lenses came later, once people got used to SLR viewfinders, and didn't mind the slightly dimmer and slightly smaller image. I think this was in Keppler's column some time back. Pat White
