According to a wiki article, a special abridged version of That Hideous Strength was published in 1946. In the wiki article about The Space Trilogy in general, this edition is described as "specially abridged by the author", C.S. Lewis. What is the difference between That Hideous Strength and this specially abridged The Tortured Planet?
In Walter Hooper's C.S. Lewis: A Complete Guide to His Life & Works, there is a section "A note on the Variant Texts of That Hideous Strength". There were three editions of the work:
Number 1 is considered the most complete and correct edition. Number 2 was set directly from Lewis' manuscript and contains some errors not found in number 1. And number 3 is the one you're referring to, and was abridged from number 1. Lewis himself performed the abridgment and also made a number of corrections.
Lewis described it thus (as WizardOz noted in a comment):
In reducing the original story to a length suitable for this edition, I believe I have altered nothing but the tempo and the manner.
Lewis also did an abridgment of Perelandra, but it was never published.
I have just come across this page as I'm re-shelving my books and noticed I had two copies of this book. In trying to decide which to get rid of I noticed they were vastly different in size and noticed textual differences.
Turns out one is the Pan abridged edition, first published 1955 - 252 pages. The other is the Macmillan edition which says Copyright 1946 but "First Macmillan Paperbacks Edition 1965" - 382 pages.
Here is an example of the differences from the first page:
Pan: Through the open door she could see the tiny kitchen of the flat and knew how tidy it was. The beds were made and the rooms "done".
Macmillan: Through the open door she could see the tiny kitchen of the flat and hear the loud, ungentle tick tick of the clock. She had just left the kitchen and knew how tidy it was. The breakfast things were washed up, the tea towels were hanging above the stove, and the floor was mopped. The beds were made and the rooms "done".
The third paragraph in the Pan edition is 4 lines long, the Macmillan edition is 16!