The code can be identified and cracked by reference to a set of copies of the privately-printed catalogues of his manuscripts, annotated with valuations in code.
Each page is annotated in the left margin with: the running-total carried-over from the previous page; a value in code against each manuscript; and a new running-total at the bottom of the page:
From this we can deduce that x = 0, because x + x + x + x = x, and x + x + n + x = n.
Also, A + R + N = 10 (or possibly 20), because when added to n, the result ends in n.
It follows from this that if A + R + N = 10, then E + 1 = R.
It does not take long to crack the code: B=1, R=2, A=3, etc., using the code-word BRAUNSCHWEIG.
Before Britain adopted the decimal system of currency in 1971 there were 12 pence in a shilling; this was based on Roman coinage, in which 12 denarii equalled one gold solidus. This is perhaps why, as a price-code, BRAUNSCHWEIG is unusual in including 12 letters (plus "x" for "0") instead of 10, as one only needs 10 letters to represent the numerals 1–9 and 0.
The question remains, however, whose price-code this was: these copies of the Ashburnahm catalogues could have been owned and annotated by almost anyone. The most obvious candidate is Thomas Brooke (1830–1908),
But the code did not belong to Brooke. Loosely inserted into one of the volumes is a folded sheet of writing-paper, using the same price-code and in the same handwriting, with the printed letter-head "Ashburnham Place / Battle, Sussex.":
Ashburnham Place was the home of the Earls of Ashburnham: