John, I can’t shed any light on Australian Mk1 and Mk2, but some years ago I did a bit of research on British 9mm Mk 1 and Mk 2.
The powder in the two is totally different. The Mk1 had a chopped cordite powder while the Mk2 had a more common looking blackish grain powder. The Mk2 hst was introduced by both HN and BE in 1944, but I have found Mk2 powder loaded in cases with Mk1 style headstamps. I was working with loose ammunition, so I don’t know if these Mk2 loads in Mk1 cases were labelled Mk2. I have few WWII boxes with the ammunition still in them. Boxes are generally not marked with either the date or the manufacturer of the ammunition inside. The 20 rd boxes with HN ammo are all marked Mk1, and I have a single HN Mk2 box (35rd) which with '45 dated cases. I have some 48 round boxes marked Mk1 but they are all empty. I don’t believe I have ever seen a 20rd box marked Mk2. I have no idea when Britain changed from the 20rd to the 48 and 35 rd labels but it appears to be in that order. The 50rd boxes were apparently introduced in the late 50s, with 35rd boxes were still in use as late as 57. There are also 10rd boxes. These were 20rd boxes with a 10rd label and a wood spacer to take up half the space. They were used to make 1250rd cases come out right. I also have 10rd and 20rd labels on 48rd boxes which are stamped “48” or in one case the “48” is hand written.
I expect the Mk2 rounds with '43 dated Mk1 cases would appear in a 48 rd box marked Mk2, but that is only a guess.
I’d like to hear from anyone with some number of 48 rd boxes with the ammo still in them to see if we can pin this down a bit better.
If anyone has a 20rd box (or better yet a 10rd box) marked Mk2, please post a photo for the rest of us.