There doesn't seem to be much difference comparing them side by side on Komplett. There seems to be an M51B too.
Samsung aren't very helpful. In one e-mail, they told me the 41B and 51B are the same apart from cosmetic differences. In another, they told me the response time of the 41B is 16ms (almost certain that's wrong). When I tried to clarify the response times of both models, they sent me a mail saying "due to the nature of your query, please call our Support Team on 0870 242 0303 for assistance." Their UK website doesn't list all their models, and different European websites seem to give different info for the same models.
They don't seem to be trying very hard to help people who want to buy their products!