Harold, I'll be interested to hear whether your Kindle version has a Preface. My library copy is the the Modern Library copy. In the beginning it says the text is that of the third edition, revised and corrected by the author in London, 1831. That would be Mary Shelley.
The reason I'm so interested in this - is that the great author and poet, Percy Bysshe Shelley,
Mary's husband, wrote the Preface to Mary Shelley's Frankenstein in September 1817. The Preface he wrote in 1817 is included in the copy I have, following the Introduction by Mary written in 1831.
The first edition is not attributed to Mary Shelley, but the Preface written by her husband is made to sound like the author of the book.
The second edition that was published in 1823 is credited to Mary. In 1831 the revised edition came out. In the Introduction to this edition, Mary writes that she has made several changes to this version - added a longer Introduction, and she says her alterations are principally those of style. She has "mended the language of the first volume."
I wonder how much influence Percy Shelley had on his young wife.
Here is the frontispiece that appeared in the 1831 edition -
