Ciphers of Elizabeth I

[John Young]

The Public Records Office search engine has one listing for "ciphers:"

  http://www.pro.gov.uk/leaflets/ri2056.htm

The citation lists "Ciphers, Elizabeth I to George III (SP 106)."

From Columbia University Library, which led us to the PRO, a 
catalog entry refers to "Cryptography, Great Britain, History, 
Elizabeth, 1558-1603, Sources," which points to a reprint of 
one of the documents listed under "ciphers" at the PRO:

  Calendar of State Papers Foreign, Elizabeth, 1558-1589, ed. 
  J Stevenson, A J Crosby, A J Butler, S C Lomas, A B Hinds, 
  R B Wernham, 23 volumes (London, 1863-1950).

I shall be looking at the Columbia University reprint of the 
"Calendar of State Papers," and will be curious about what
is in "Ciphers, Elizabeth I to George III (SP 106)."

My question is whether these documents have been examined
by persons here and/or whether the Elizabethan era ciphers 
described in them have been written about in contemporary 
works.