My Air Europe Pal's reply ---- >>>> Crikey Roger, I just knew them as the 535C and 535E4 models.
Have a look at this EASA Type Data document might help you >
https://www.easa.europa.eu/en/downloads/7699/en
FYI --- Sorry, but that does not refer to 12B or 21B, but on some other AD Docs viewable onlineit does,
but does not differentiate the designation as such of what it means.