I was recently made aware of a digitized manuscript at Princeton from the collection of Henry Yates Thompson, which has his characteristic "Ex Musæo" label inscribed with his manuscript number, price in code, source and date of acquisition:
In these publications, Henry Yates Thompson's price-code has been given as BRYANSTONE (which is self-evidently wrong, as it contains the letter "N" twice), on the basis that he lived for a time at Bryanston Square, London. But in addition to the problem of the double "n", "Bryanston" is spelled without an "e".
The correct code-word is BRYANSTOLE, and the inclusion of the letter "L" is proven by its use in the St-Omer Psalter at the British Library (Yates Thompson MS 14), where "lee.e.e" = £900 0s. 0d.:
EDIT, 26 March 2016
I recently realised that the code is incorrectly given as BRYANSTONE in Frank Hermann, Sotheby's: portrait of an auction house (London, 1980), p.190 n.1.
Laura Cleaver has kindly directed me to this image, from which these are screenshots:
"Yates Thompson's personal cypherWilliam Henry Bond was a scholar-bibliographer who served as the Librarian of the Houghton Library from 1964-1982.
was "Bryanstole" (1-10) W.H. Bond 10/6/52
["sen", inscribed on the bookplate] = 605"
Christopher de Hamel, Hidden Friends: a Loan Exhibition of the Comites Latentes Collection of Illuminated Manuscripts From the Bibliothèque Publique et Universitaire, Geneva (London, 1985), unpaginated page facing nos.28–29.
Christopher de Hamel, 'Was Henry Yates Thomson a Gentleman?', in R. Myers and M. Harris (eds), The Property of a Gentleman: The Formation, Organisation, and Dispersal of the Private Library, 1620–1920 (Winchester, 1991), p.83
Christopher de Hamel, Gilding the Lilly: a Hundred Medieval and Illuminated Manuscripts in the Lilly Library (Bloomington, 2010), p.126.
[Ian Jackson], The Price-Codes of the Book-Trade: A Preliminary Guide (Berkeley, 2010), p.9, citing de Hamel 1991 as his source; a second edition is in preparation, in which this error is corrected.