Alright. After 3 attempts at this pattern, I think I've finally got it!
Attempt 1: Didn't have the recommended 4/5 oz veg tan leather plus lining. Used instead some 6/7 oz... Turned out to be too thick (surprise!). The leather used was scrap belly leather, and didn't have consistant thickness and pliability... Decided I didn't want to fight with it, so I threw it back into the scrap bin.
Attempt 2: Cut each piece twice, from 2/3 oz veg tan leather, laying them together, smooth side out. The thickness was good, but when I started putting it together, I realised the pattern itself wasn't symmetrical, and so the case wasn't turning out symmetrical and things weren't lining up where they should... So I scrapped that attempt. Thank goodness this is a hobby and not something were I need to be making a certain $ per hour.
Attempt 3: Traced the pattern again on paper, folded it down the middle and averaged out places that were not symmetrical. Glued the pattern to cereal boxes, cut them out, etc... Pieces were cut, dyed, sewn, etc. When all three pieces were complete, and triple checked for mistakes along the way, I decided on how I would put the middle divider in. There were no instructions on how exactly this divider went in, except for the pictures in the article photos (not on the pattern itself) that showed where some holes had been punched in the divider. Using this as a reference, as well as a friend who is an Enfield enthusiast, I decided on how I would put the divider in. I put everything together with copper rivets, and put the 2 Enfield clips in. Turned out the middle divider was too long, and wasn't behaving as it should when the clips were in it and both flaps were closed. Only solution I found was to take all the copper rivets out and try to put the divider in differently - which I did.
Pouch now closes similar to the picture, although it still seems to be about 50% too wide for the Enfield 5-round clips. Also, I wasn't able to recreate the way it buttoned closed. The article author made his own hardware for it, since he didn't have anything that would work. I chose to just use what I had, but be a bit more creative with it.
Also, my tiny little 11 lb anvil couldn't reach far enough into the pouch to set the copper rivets at the bottom, so I had to use Chicago screws... They look terrible and were a nightmare to put in, but they serve the function I need!