I don't know how it got past the editors, but the secret of the notation of A Concordance to the Chuang Tzu, Harvard-Yenching Index Series No. 20, has been leaked by James Sellmann in his "Transformational Humor in the Zhuangzi" (Wandering at Ease in the Zhuangzi)! 45/17/87-91! Page, chapter, line(s)! So, this particular notation refers to page 45 (of the concordance), which corresponds to the 17th chapter of the Zhuangzi, lines 87 through 91. Amazing. If only we had the concordance, it might be quite helpful.
Ziporyn uses his own notation (for the Inner Chapters only) for the purpose of referencing to comments of the commentators. When quoting his translation, I use these when possible. Mair has numbered the various stories, anecdotes and ponderings of which the chapters are composed and thus one may refer to 17:7, which is that passage signified in the numbers given above: "The Happiness of Fish", as I call it. When quoting Mair's translation I use this.
Despite my enthusiasm at now having the key to the notation that scholars use, not having that concordance, it probably does me little good. And it's probably in archaic Chinese, in any case. Oh well, at least I now know to which chapter they refer.
As for the story about the happiness of fishes, I have written about it more than once, though never with a sense having fully understood it. It is dealt with at least twice in “Wandering at Ease in the Zhuangzi”, by Ames (“Knowing in the Zhuangzi”) and by Sellmann, and though both have helped to enlarge my understanding, still I find it a bit opaque.
Zhuangzi and Huizi are strolling on a bridge over the river Hao when Zhuangzi, seeing the minnows darting about, comments that this is what fish enjoy doing. Huizi asks him How he, not being a fish, could possibly know what fish enjoy. Sellmann points out how Zhuangzi’s answer contains two points of humor. The first is seen in his remark that in asking How he knows, he is in fact inferring that he does know. And in any case, he is inferring that he, Huizi, can know what Zhuangzi does not know, and thus indirectly confirms that it is possible for Zhuangzi to likewise know what fish enjoy.
The second point turns on the word translated how which is actually a particle that introduces a question of what, where, when, or how. Zhuangzi answers not how he knows, but from whence he knows: “I know it here on the banks of the Hao River.” All our knowing takes place within the context of our unique situation; every point of view is from and determined by a particular situation.
Ames makes the point that all knowing is a relationship between the knower and the known; there is no objective knower who knows an objective other. Zhuangzi knows the enjoyment of fish because he knows his own.
You can check out Scott's writings on Zhuangzi here.