in those photos I want to point out some key things you may be discounting or don't see. As a caveat, don't have sheaths in hand so going by photos and can't enlarge with any clarity.

1)The RMK marked sheaths have different stitching than the Heiser marked sheaths particularly second from left. One to right seems more the similar as HHH marked sheaths.
2)The stone pouch flap lacks the tooled edge common on Heiser marked sheaths
3)The "butterfly" is different, with the tell-tale extra holes that were so common on Johnson sheaths
4)The front stitching appears to lack the "tooled" edge of the Heiser marked sheaths
5)The shape of the two RMK marked sheaths is a bit different than the HHH stamped sheaths and most noticeable in the one second from left
6)The leather of the two RMK marked sheaths appears different from front and back views.
7)The backstitching at the throat of Johnson's always seemed a bit cruder than on Heiser products.

Now without having in hand, a true assessment is difficult but some of these points made are valid. Obviously none of the sheaths from the same maker are going to be 100% exact to the next one, but there is a reason for differences beyond "normal" variations during production.

to your points:

1)Early JBB had center position. Change was made to improve holding power of "anemic" BB
2)true, why I brought it up about the two RMK stamped sheaths
3)I do not believe every HKL RMK stamp was "west facing". Perhaps the vast majority, but not all.
4)More or less but look at the two RMK sheaths relative to what you say in this point

Here are two more photos to help with the assessment of the sheath in question: