I've just been leading the MRG on the Lewis/Blackburn exchange in this collection concerning whether quasi-realism is fictionalism (Blackburn's half of which is online here).
It seems to me that Lewis's claim that quasi-realism is a version of fictionalism is mistaken, but not for a reason that comes through in Blackburn's response.
Lewis's argument, in essentials, is that the quasi-realist wants to say everything the realist does, so must either be a realist or be making believe that realism is true. But he is not a realist, therefore he is making believe that realism is true, i.e. he's a fictionalist.
I think we can show what's wrong with that argument in a few lines. The sense in which the quasi-realist 'says everything the realist does' is not the one which enforces either being or pretending to be a realist. This would be enforced if the quasi-realist was making the same assertions as the realist, but he isn't (indeed, the paradigm quasi-realist isn't making assertions at all). The quasi-realist says things which sound like what the realist says, but they are to be interpreted differently - in the moral case, as expressions of attitudes, rather than as committing to moral properties. Expressing an attitude requires neither belief in moral properties (realism) or pretense that moral properties exist (making believe that realism is true).
So here's why quasi-realism is not fictionalism:
The fictionalist differs from the realist in adopting the realist account of the meaning of the target sentences but dissenting from those sentences (same content, different attitude) whereas the quasi-realist differs from the realist in adopting a different account of the meaning while continuing to accept those sentences (different content, same - or at least similar - attitude).