Seven of the last ten books have a suffix attached to the number; in six cases "X", in one case "J". I have a plausible explanation for the part I change but this one stumps me. Why these seven and not the other three books? Why only one "J"? Anyone have any ideas?
The books without a suffix are 536, 538, 540.
The ones with a suffix are 535, 537, 539, 541, 542, 544, 545.
There is no book with the 543 number.
Here's one last thought. Two of the books without a suffix (536, 538) are short westerns with no price. The six "X" books have a 35 cent price. The one "J" book has a 50 cent price. The only book that doesn't fit this pattern is 540 with no suffix but marked with a 50 cent price. Perhaps that's an oversight and the suffix did equal the price?