The first letter indicates the year of manufacture, so K = 2016, M = 2017 and N = 2018.
The numbers indicate the spec of the TV and the higher the number, the higher the spec.
Hope this helps.
BTW. I am looking for a 40". Which one would you recommend?
Hi @Binnenstraat, I would agree with @AndrewL that the MU6400 offers good value and if looking for a 40 inch should suit your needs. Thus review places it highly for a smaller UHD set https://www.whathifi.com/best-buys/tvs/best-40-43in-tvs You can see more detail on specifications here https://www.displayspecifications.com/en/model/d61db42 The equivalent in the NU series does not offer bluetooth support .