Actually, the authors stated very clearly why they assigned ‘A’ to 27 and gave explanations for the ‘&’ and ‘E’. They describe their discoveries in Secret Shakespearean Seals, 1916, on page 28, and with Plate XXXIII.
The following link leads to a scan of the book which it now seems must be downloaded before you can see the whole pages.
The Resuscitatio was published by Bacon’s Chaplain, Secretary and Confident William Rawley. Actually, the strip of paper said “twenty-seventh Folio.” They noted that there was no known “Dr. A” or anything about a “27th folio” but these fit well the concept of a Kay cipher that they were trying to unravel. It made sense that the letter ‘K’ was the numeral 10 but then why wasn’t the letter ‘A’ equal to 25 since ‘Z’ was 24? Testing with ‘A’ as ‘27’ yielded some promising signature candidates. Here is the passage that they found:
And finally, in the same book on “Shakespearean Seals”, pgs. 63-65, the authors describe how they found an old book “The Repertorie of Records” from 1631. It was published anonymously but the authors say that it has been assigned to Thomas Powell, who wrote the Attourneys Academy. There are some references to places connected to Francis Bacon (such as Yorke-house and Saint Albans), along with a trail of significant numbers (like the number 33 which is the Simple count for ‘Bacon’) which led them to a page describing the “27th chest” as ‘a’ and the 28th as ‘b’, while the 25th was listed as ‘&’ and the 26th as ‘E’ (probably for our ‘et’). This along with the Resuscitatio discovery led them to revise and test the Kay Cipher alphabet they were working on.