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It's the page-breaks from the first two editions, so you can make sense of English-language texts that refer to those editions. See page ix in the Editorial Preface to the Fourth Edition.
I had this same edition out the library recently and was wondering the same thing (but not hard enough to actually, like, look for the answer). Nice to know what those were about!
It struck me as the kind of thing that would be explained in a preface or table of notation or something, so I had a quick look. As you say, nice to know the answer :-)
I think the thing that caused me to miss it was how early it was mentioned in the preface. I somehow expected notation to be explained later on.
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