Originally posted by steve rowell
The last two letters are Boeings Customer designator, obviously the aircraft was built for someone else other than KLM

It was built for KLM! They just decided to order the same spec as Transavia for several reasons. One of which has been mentioned by tenthije, another being the fact that Transavia and KLM sometimes exchange planes when schedules demand this. On the -300s and -400s this meant extra pages in the KLM manuals for the Transavia aircraft etc. This is now not needed anymore.