Maybe the k are all the left over low yield chips that Bitmain wants to get rid from their inventories before ceasing S9 sales?
The K version uses BM1393 and BM1393B chips, S9/S9i use BM1387 and BM1387B and S9j uses BM1387BE, I am not sure if these "models" indicate different chips from TSMC itself or Bitmain adds something more to them after receiving them from TSMC, in other words, not sure if these are the worst chips Bitmain has or TSMC sold, this is due to the lack of my understanding on how these chips are actually used by bitmain.
Having said that, you are probably right, and it does not matter how did these chips came into existence, they are simply the worst once out there. I keep losing a board every now and then, to the point where I stopped bothering, these S9ks run alongside with old S9js/S9s that have been mining for years without any major issues, I think the Chinese know something we don't about the K version, by looking at the price tag in the retail market in China, a used S9j goes for almost the exact price as a brand new S9k.
