Thanks Len44, ttocs, BDH55.
Based on your inputs, the Renaissance ESL models use a new 13 Characters coding, which started in 2016.
Not clear if this new numbering scheme started for all new loudspeakers manufactured after 1/1/2016 (or some other calendar date), or if it started with item #1 of a new model type (Renaissance ESL ?) and then spread as new types were introduced. From a manufacturing point of view, the first would make more sense.
AFAIK it seems to be something like : AAB9999999C88
Meaning is not obvious, could be something like :
AA -> model family type -> AF : Renaissance
B -> ???manufacturing year???
9999999 -> ???manufacturing date & unit number???
C88 -> ???appearance???
Note that some S/Ns found on the Internet do not follow at all this supposed numbering scheme.
For instance this
Renaissance ESL 15A : RENPP005CES16.
Martin Logan could also have decided on "random" S/Ns, departing from previous schemes, which had meaningful group of characters (letters or digits).
If somebody has more information or better suggestions...
PS : it would be SOOO MUCH SIMPLER if Martin Logan provided at last an "official decoder", possibly as an internet link.