Great photos and great info as per usual from you.
A couple of observations on my part.
Notice the heavy roll crimp on the M43. That is because the brass bolt (rod) was NOT supposed to enter the case mouth but was supposed to fit on top of it. The MK I appears to be made so that the bolt enters the case.
MK I are headstamped 45-70. Since most of the WW II line throwing guns were Trapdoors with original chambers and bores I suppose it seemed natural to headstamp them thus. Later guns like the H&R had special made barrels and I don't recall them being marked for caliber.