Research Notes:
French cipher disk in silver by Nicolas Bion (1652-1733).
Maker of mathematical instruments to King Louis XV.

All material is Copyright (C) 2011 by Nicholas Gessler or by the owners of the images.
Posted 14 October 2011, revised 31 October 2011.

This silver disk turned up in a small antique shop in Southern France and was subsequently purchased on eBay in 2006.

The 24-letter alphabet omits "J" and "U" and procedes clockwise in alphabetical order.
In the outer circle, the numbering corresponds with the lettering except for "A" through "F."
In the inner circle, the numbering has considerably less correspondence with the lettering.

Nicolas Bion's design corresponds exactly to the illustrations Schotti (below).
Since Schotti's illustrations appeared when Nicolas Bion was 13 years of age,
we can assume that they were the inspiration for Nicolas Bion's device.

"N Bion A Paris"
Diameter is 4.5cm or 1.77inch.

There is a wide range of workmanship in Nicolas Bion's instruments as evidenced by the
signatures of these devices. Consequently, it is reasonable to assume that they were produced
by several people under his employ. This signature, however is better executed than most of the
others which might lead one to suppose that it was made by Nicolas Bion himself for an important
client, perhaps his contemporary King Louis IV (1643-1715) or his cryptographer Rossignol.

"Figure 1:" A disk identical to Nicolas Bion's cipher disk in both numbers and letters.
P. Gasparis Schotti, SCHOLA STEGANOGRAPHICA, 1680 edition (Gessler collection), figure 1, page 95.
Apparently, the same plate was used for both editions as evidenced by the flaws at 11 o'clock.

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Left: The fronticepiece of Schotti's SCHOLA STEGANOGRAPHICA.

Right: "Figure 2:" A disk similar to Nicolas Bion's but with a different association between numbers and letters.
P. Gasparis Schotti, SCHOLA STEGANOGRAPHICA, 1665 edition (Library of Congress), figure 1, page 99.

Outer disk letter sequence in numerical order: FBCDE AGHIK LMNOP QRSTV WXYZ
The initial RED sequence of 7 letters is an irregular progression, but the remaining letters are in alphabetical order.

Inner disk letter sequence in numerical order: DBCFE HGKIM NLQOP TSRWV AYZX
The BLUE sequence is an irregular progression.

Executable - Source Code - Zipped Project File

It seems likely that the progression served as a keying method:
Rotating the BLUE inner disk so that it's number "1" aligned with the number "1" on the RED outer disk (which served as an index) gives the first alphabet for keying.
Rotating the BLUE inner disk so that it's number "2" aligned with the number "1" on the RED outer disk (which served as an index) gives the second alphabet for keying.
Rotating the BLUE inner disk so that it's number "3" aligned with the number "1" on the RED outer disk (which served as an index) gives the third alphabet for keying.
If we continued this, using a separate alphabet for each letter in the cleartext, we would have a key of length 24 letters.
Using the convention that "A" on the BLUE inner disk serves as an index, and points to the indicator on the RED outer disk, we get this progression or key:
CEDAB YZWXT SVPRQ MNOKL FHGI
Using the convention that "A" on the RED outer disk serves as an index, and points to the indicator on the BLUE inner disk, we get this progression or key:
YWXAZ CBEDG HELIK ONMQP VSTR

We could then repeat the key, but shifted, in the following manner:
Rotating the BLUE inner disk so that it's number "1" aligned with the number "2" on the RED outer disk (which served as an index) gives the twenty-fifth alphabet for keying.
Rotating the BLUE inner disk so that it's number "2" aligned with the number "2" on the RED outer disk (which served as an index) gives thetwenty-sixth alphabet for keying.
Rotating the BLUE inner disk so that it's number "3" aligned with the number "2" on the RED outer disk (which served as an index) gives thetwenty-seventh alphabet for keying.
If we continued this, we would now have a key of length 576 letters.
The close clustering of the progression on the BLUE inner disk would serve to simplify its manipulation.
The initial irregularity of the RED outer disk would serve to hide its function as an index.

In a similar manner we could do the inverse, but it would not produce as irregular a key:
Rotating the RED outer disk so that it's number "1" aligned with the number "1" on the BLUE inner disk (which served as an index) gives the first alphabet for keying.
Rotating the RED outer disk so that it's number "2" aligned with the number "1" on the BLUE inner disk (which served as an index) gives the second alphabet for keying.
Rotating the RED outer disk so that it's number "3" aligned with the number "1" on the BLUE inner disk (which served as an index) gives the third alphabet for keying.
And so on...
Using the convention that "A" on the BLUE inner disk serves as an index, and points to the indicator on the RED outer disk, we get this progression or key:
CYZAB SDEFG HIKLM NOPQR STVW
Using the convention that "A" on the RED outer disk serves as an index, and points to the indicator on the BLUE inner disk, we get this progression or key:
YCBAZ DXWVT SRQPO NMLKI HGFE
Unlike the previous method, this keying system is largely regular, mostly shifting the disk clockwise or anticlockwise one step at a time.

Is this too complicated a procedure? Alberti suggested using a short key in the 1400s. The Rossignols were practised in cryptanalysis. By comparison, this procedure is relatively simple.

Time-Line of personages and publications.

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Left: Nicolas Bion. Right: Antoine Rossignol, King Louis XIV's cryptographer and founder of the "Chambre Noir" at Versailles.

XXX
King Louis XIV of France, "The Sun King."

Works signed "Nicolas Bion" were not all done by the same hand. These two pieces are among the most masterfully executed.
Such quality was likely reserved for wealthy patrons and/or royalty, suggesting that this cipher disk may have been made for Antoine Rossignol or Louis XIV.

Note the nearly identical lettering, "leafy flourishes" and swirling motives between the silver cipher (left) disk and the silver sundial (right).