Horizontal RMK logo = early Johnson sheath. A beautiful equation.
Heiser may have changed a bit with the stone pouch flap and such, but still recognizable as a Heiser nonetheless. There is no dispute there, but the one's that seem to be in question, well, there is a question. There aren't many unmarked sheaths from the period you mention, not enough to make a claim as to who made them. I know some are for sure not Heiser and not Johnson. We are talking 1959-1960. Then there are one or two I have seen that I could make a case they might be real early Johnson samples. Stockman is out as from what I have seen, his sheaths are readily recognizable.
I will go back to the fact the Johnson was in the area in the late 50's after his retirement as a police officer. Perhaps one day there may be some light shed on this matter, but until that time, RMK marked sheaths will remain Johnson sheaths for my purposes.