A little information on "It's definitely not a K" ... when you are a bit more into perfins, you discover the most weired things perforated. One letter inverted and almost exactly on the other holes, even two different perforations on one stamps are possible.
And there have been a lot of punching machines in use with broken/missing pins. So even if there is just an "I" visible, it could be something else.
For identifying take the letters that are obviously correct, and see what can fit to the vertical line -> B, D, E, F, H, K, L, M, N, P, R, T (in some cases).
Here is a wonderful example of missing letters in an Austrian perfin, that is found in 99 % incomplete.
You might think of an American company -> US/TF (last picture), but if you are very lucky, you find a stamp with the complete perforation. Then it reads -> FLUSS/HUTFB for Fluss Hutfabrik in Freiberg.