The JR7-K variant is completely obsolete now. Every possible build can be exactly duplicated with a JR7-D. However, the -D can run builds that are not possible for the -K. This was also true before the module change, but the -K used to have an extra module slot to make up the difference. The -K and -D are unique in that they have this relationship -- there are no other two mechs where one can replicate all possible builds of the other while also having extra hardpoints.
[edit to add: I missed the Stalker 4N, but since that is overlapped by TWO mechs and not just one, this is still technically true. Sorta.]
Maybe it would help to have a Venn diagram. Draw a circle. Then draw a larger circle around the first, surrounding it completely. The small circle is the set of all possible -K builds. The larger circle is the set of all possible -D builds. Again, this relationship is absolutely unique in the game.
[edit to add: except for the Stalker 4N]
The JR7-K should have it's extra module slot restored for this reason. That would leave the -K and -O as having one extra module, while the -F and -D would have the standard loadout of modules.
Edited by Ertur, 26 August 2014 - 10:55 AM.