(shrug) I only know my own path.I'll bet barber supply houses never sold barbers magnifying tools for razor sharpening. What did they know that we didn't? Was it a case of ignorance is bliss?
I can imagine, though, that the kind of experience a professional would accumulate would be a very reasonable substitute for magnification, eventually. I also have a vague impression that barbers did not routinely dish out the kind of awesome shaves we achieve through our obsessive geekiness.