HI there. It certainly has the appropriate straps on the back for vertically mounting on the brace, doesn't it?
I haven't seen any examples with the adjustable strap on the outside. Interesting. Maybe it's for closure when empty or full? I wonder if the small tab on top is for attachment to the buckle at the front of the brace so the pouch is held at a consistent height above the main pouch?
In the pictures and artwork I have
seen, there isn't a strap but rather a simple flap with a noticeable angle to it to accommodate the magazine upside down (I would think)..
Might you obtain measurements? I can provide those for the Mk I mag to see if they match...
They were introduced with the equipment in 1888 but were withdrawn in 1890 LoC 6233 (and the spare magazines LoC 6235) which would explain their rarity.