What is significant about the description of the letter found by the narrator?

The narrator's discovery of the Scarlet "A" provides the basis for his semi-fictional account of the story behind the Scarlet Letter. Upon reading a brief description of it, the narrator decides to write his book.

"This rag of scarlet cloth,--for time, and wear, and a sacrilegious moth, had reduced it to little other than a rag,--on careful examination, assumed the shape of a letter. It was the capital letter A. By an accurate measurement, each limb proved to be precisely three inches and a quarter in length. It had been intended, there could be no doubt, as an ornamental article of dress; but how it was to be worn, or what rank, honor, and dignity, in by-past times, were signified by it, was a riddle which (so evanescent are the fashions of the world in these particulars) I saw little hope of solving. And yet it strangely interested me. My eyes fastened themselves upon the old scarlet letter, and would not be turned aside. Certainly, there was some deep meaning in it, most worthy of interpretation, and which, as it were, streamed forth from the mystic. " Custom Houe The Scarlet Letter)