All I know is the old chisels I bought when I started restoring my wooden canal boat were Sorby, Collins or Marples and all had "Cast Steel, Sheffield" on the tangs and the metal was/is a very dark colour. They are superb chisels!
When I was a clay modeller for the car industry, many of my fellow modellers were ex patternmakers from the old car industry in Coventry and all swore by the above makes, especially the first two. These can be found for a quid each at Sunday markets and boot fairs and I've often seen turning chisels there. I sometimes wonder why we couldn't use ordinary chisels for finer turning. Do they HAVE to be that particular pattern to use at a lathe?
Martin