Well, Mike is very modest. He, being the horn repair master makes it look easy. That notwithstanding with some moderate effort and the right method horn can be miraculously transformed. I learned most if not all of what I know about it from Mike and the "What Are You Working On" thread.
There are a couple of reasons why those razors look similar. Part of it is just a style. You notice how looking at cars from the 30s they all look very similar and so do those from the 40s, 50s etc. There's a certain paradigm that manufactured goods follow at a specific time. The other reason is that many of those grinders were taking in contract work. A grinder might work primarily for Wade and Butcher but might also take work in on the side from a smaller company, or someone who was not yet a free man may get some work on the side that his master let him take if he was benevolent. So the same grinder who made Wade and Butcher might also make eyewitness or James Johnson razors. At least this is my understanding of how that worked in Sheffield from what I've read. I do know for sure that it was that way in Germany. ERN ground most if not all of the razors for J.A Henckels at one point as well as producing their own.