I have a AC701 (no K) inside a Schoeps M221 I just got. I don't know if the mic is working yet as I have no PSU for the moment.
If I read the primer correctly, it seems to be a post 1965 tube. And if I read correctly, it does not necessary mean that this is a non selected tube or a switched out one right? In other words, this might be the original Schoeps factory tube even if there is no "k", right?