Yet a stimulation must be conceived for these purposes not as a dated particular event but as a universal, a repeatable event form. We are to say not that two like stimulations have occurred, but that the same stimulation has recurred ... Certainly it is hopeless nonsense to talk thus of unrealized particulars and try to assemble them into classes. Unrealized entities have to be construed as universals.
These comments seem to undermine the point of his entire enterprise of elaborating meaning and synonymy through the vehicle of "stimulus meaning." He begins in Chapter 1 with the claim that we learn to associate verbal expressions with stimuli through social reinforcement. Consistent with the comment above, that process would be utterly ineffectual without a built-in mechanism capable of abstracting and recognizing patterns. Thus I am in agreement that this process is essential to both learning and representing the sentence, even though the details of the process remain mysterious. As pointed out in my critique of Chapter 1, the existence of such a process suggests that the social reinforcement element is vastly less important in the process of assembling a meaning, and merely assigns a term (or sentence).
But it also then becomes unclear what work is done by his intermediary notion of stimulus meaning, aside from adding complexity. The method depends entirely on this unspecified abstractive process: to determine whether an actual and particular stimulus will prompt assent, we must either fully understand this process of abstraction and abstractive pattern-matching, or be able to actually perform it. A translator can of course perform the process, though it is entirely possible that he will perform it differently; one of things we do know about such abstraction is that it is dependent on salience to the individual. In any case, instead of selecting an arbitrary and completely unfamiliar notion of a "repeatable event form," it would seem to make more sense to allow that the universal is simply the abstraction thus created. The repeatable form is exactly that which triggers the matching of the pattern of the stimulus against the sentence, and thereby prompts affirmation.
The insertion of this patina of set-theoretic empirical terminology explains nothing. It is entirely dependent on an undefined mental process that is the real source of the "meaning" of either a sentence or a term. Perhaps an explication of this issue is forthcoming later in the chapter or in later chapters. My suspicion is that Quine's approach is an attempt to connect behavior to logic, which might be viewed as a more rigorous treatment. But abstraction is the connection of behavior to logic, and assuming it without explanation avoids the most important topic.