Sheffer stroke the most important advance in logic? I think I once read, or heard, that Bertrand Russell once said that the discovery that all logical operators are expressible in terms of the Sheffer stroke was the most significant advance in logic since the publication of Principia Mathematica, and that had he and Whitehead known about it beforehand, they would have proceeded completely differently.
This is a really strange claim, so I imagine that I (or the person who told me) misunderstood, or misheard, or made it up completely.
If Russell really did say something like this, what exactly did he say, and where and when?

The focus that Russell had was very different than the focus logicians have today.  Russell's viewpoint of "logic" was tightly connected to Principia Mathematica, and his interpretations of other results depended on their relation with PM, because that system was the basis for his research program in logic and philosophy. 
Russell's work was the peak of the logicist program: to argue that all of mathematics can be expressed in pure "logic" and to study the properties of that "logic".  The existence of a single operator to which all other "logical" operators can be reduced would be wonderful from that point of view - just as a single physical law that explains all of physics would be wonderful for the foundations of physics.  The use of a single primitive would have reduced the number of "logical notions" that were needed in the foundations of PM, which was an important philosophical goal.  
So, in the context of PM, Russell's remarks were perfectly reasonable. The key is that his viewpoint of "logic" was directly focused on PM. 
