Each alphabet takes on the character of the tool that constructed it. These calligraphic letterforms were made with a broad, flat tipped tool held at a 45 degree angle. Such letters have been made by a reed pen, a quill pen, or a steel nib. But in this case he's using a rigger brush held at a 45 degrees.
Gordon classifies such an alphabet under the broad category "text" by which he means the old-style or thick-and-thin letters made through the centuries by normal writing instruments.
Finally, I would like to thank blog readers Rise of the Molecule and John Berkeley for letting me know about the following related videos.
(Direct link to video) Factory tour of the Speedball pen nibs.
..and John told me about this video about the traditional fairground lettering and decoration of Joby Carter.
Amazon book:The Lettering and Graphic Design of F.G. Cooper (Thanks, Bill P.)
Free book: Lettering for Commercial Purposes, William Hugh Gordon, 1918
Read the whole series:
Show Card Writing, Part 1
Show Card Writing, Part 2
Show Card Writing, Part 3