(from PW18 tracking)
"argent de billon" = a coin of alloyed silver in which there is less silver than other metal.
Left in Fr; is there an English trans?
See p119v_2
"billon" is in the OED, with instances from the 18th c.
Suggestion: translate as "billon silver" and add ed. note "Billon refers to a mixed metal used in coinage, consisting of gold or silver with a preponderating admixture of a baser metal. (from OED)"