Collins White Circle Modified Numbering Part II

Part I discussed a change to Wm Collins Sons & Co. Canada Ltd's numbering for their White Circle imprint that affected the last four years of the run. There was one other change at the tail end of the run in 1952.

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?