The ars article shows an image of an original encryption table used in some other letters by MQS. It's a quite detailed manual. The researchers apparently did c

The ars article shows an image of an original encryption table used in some other letters by MQS. It's a quite detailed manual. The researchers apparently did consult that and other known tables in their analysis. Also mentioned, "... The authors noted that seven of the eight newly decoded letters turned out to have plaintext copies preserved in British archives, adding, "It would seem that the leak from the embassy was quite effective and comprehensive" ..." , as there were some spies at the time in the French embassy that were leaking the decoded texts. Btw, there's doi link at the end of the ars article.