When print publishers publish a book, they usually include a number line on the copyright page. It usually starts
10 9 8 7 6 5 4 3 2 1
And with each reprint one of the numbers gets removed (which was an easy task even in the days before computers).
So the 30 29 28 are just there for later. The next printing would have 30 90 28 instead of 30 90 28 27.
Collectors usually want first edition, first printing.
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