Just for info, although KLM usually schedule a 747-300 combi on the KL644 from JFK, on Sat 1st March I flew on PH-BFO(a 400 combi) from AMS to Tokyo and the previous operated flight was the the KL644 from JFK to AMS.
I have sat 3/4 times in the "select seating area" between rows 9-18. As stated you need full Y ticket or Elite status FD card to reserve one of these seats. (I last sat in 11D)However, you can request a seat and will get it if not reserved by someonelse.

Matt