Most illustrations in the 1898 St. Nicholas Magazine serial are legibly signed "Wm A. Mackay" with differences in punctuation
The illustrator is credited as "W. A. Mackay" in the Contents of magazine Vol 25, Part 1 (p. VII of the hemi-volume as bound), and may not be credited in later publications
Credited as "William A. Mackay" by some online booksellers