Here are two examples of Walter Johnson's signature from a similar time period to compare. Both of these are from my personal collection.
FWIW, I believe the Huggins and Scott signed photo to be an authentic example of Walter Johnson's signature. It's obvious that Walter changed the formation of the "W" and the "J" in his signature during the mid-to later 1920's, but many characters in his handwriting and the way he formed numerical characters remained fairly constant.
The first is the exact same photo image as the one posted at the beginning of this thread signed and dated by WaJo on June 15, 1927. This was signed right before Walter retired as a player at the end of 1927.
The second exemplar is a March, 1929 GPC questionnaire that was signed and filled out by Walter. This was signed just as Mr. Johnson was about to launch his career as a baseball manager.
You be the judge...