The Zodiac Killer had certainly made the figure eight the centerpiece of his coding on April 20th 1970, not only using three, but circling them as if to imply greater meaning. Was the circling of the number eights a hint towards the technique used in the coding? When we look at the first paragraph of A Few Words On Secret Writing, it features a technique using two wooden cylinders (scytala) as a method of coding, but directly underneath the example of coding in the essay, where the alphabet is split A to M, and N to Z, lies the important paragraph It states;
We know the Zodiac Killer often drew his inspiration from deep history, such as the short story The Most Dangerous Game by Richard Connell, first published on January 19th 1924, and The Mikado (1885), so what if he took this concept of two circles with a common center and looked for a cipher technique he could plagiarize in his April 20th 1970 letter. But more importantly, a technique of coding that had previously featured three eights (888) in its design.
A cipher disk is an enciphering and deciphering tool developed in 1470 by the Italian architect and author Leon Battista Alberti. He constructed a device, (eponymously called the Alberti cipher disk) consisting of two concentric circular plates mounted one on top of the other. The larger plate is called the "stationary" and the smaller one the "moveable" since the smaller one could move on top of the "stationary". The first incarnation of the disk had plates made of copper and featured the alphabet, in order, inscribed on the outer edge of each disk in cells split evenly along the circumference of the circle. This enabled the two alphabets to move relative to each other creating an easy to use key. Rather than using an impractical and complicated table indicating the encryption method, one could use the much simpler cipher disk. This made both encryption and decryption faster, simpler and less prone to error. Wikipedia.
The single and triple 8 number is featured heavily within the design of the Union Cipher Disk, with the 888 representing "tion".
Not only did the split alphabet of Edgar Allan Poe's 13 characters appear in the Zodiac code of April 20th 1970, along with the near date of April 21st, but the concentric circle cipher technique explicitly detailed in A Few Words On Secret Writing, can be found in the American history of the Union Cipher, with the number 8 front and center of everything, including the number 888.