wow, I had no idea...

I just dug out my old Hardy Boys hardcover books from way at the back of the book shelf (behind the Saberhagen Swords books) and sure enough, mine were all from 1959.

Here's the inscription from inside th cover of book #1 "The Tower Treasure"

  (c) Grosset & Dunlap, Inc. 1959
  all rights reserved
  ISBN: 0-448-08901-7 (Trade edition)
  ISBN: 0-448-18901-1 (Library edition)
  
  In this new story, based on the original of
  the same title, Mr. Dixon has incorporated
  the most up-to-date methods used by police
  and private detectives.

Unlike the book referenced in the article, mine begins

  Chapter I
  The Speed Demon
  
  Frank and Joe Hard clutched the grips of their motorcycles
  and stared in horror at the oncoming car. It was careening
  from side to side on the narrow road.
  
  "He'll hit us! We'd better climb this hillside -- and 
  fast" Frank exclaimed, as the boys brought their
  motorcycles to a screeching halt and leaped off.

A little later comes what I believe is the scene referenced at the start of the article:

  Frank chuckled and sad, "After the help we gave Dad on his
  latest case, he ought to set up the firm of Hardy and
  Sons."
  
  "Why not?" Joe replied with a broad grin. "Isn't he one 
  of the most famous private detectives in the country? And
  aren't we bright too?" Then, becoming serious, he added,
  "I wish we could solve a mystery on our, though."

Now I'll have to try and dig up a couple of the originals just for fun. I loved these books when I was 10~13... practically inhaling them one after another.