Thanks for the wonderful post and information, Gus!
LT, (May I assume that is short for Lieutenant or perhaps your initials?)
You are quite welcome, so now let's get into some more technical info you requested.
Oh and before I go further, I'm not trying to be a know-it-all, but I'm using the common 18th century terminology. Cartridge Box is a term that didn't begin to be commonly used for a pouch with a shoulder strap until the AWI or later. Their term more commonly was a Cartouche or Cartouch (various spellings) Pouch in the period and especially Pre AWI, though sometimes/less frequently written as Cartridge Pouch. What we today call a "Belly Box" for Cartridges was called a Cartouche Box in the period. I figured since you are interested in making accurate repro's, you might enjoy the more common period terms.
Oh, another caveat, if you are planning on joining a re-enactor unit, I advise you to ask if they have one or more "unit designated" patterns for their Cartouche Pouch, which some units have.
As you can see from the originals, there were a number of methods used to construct Militia Cartouche Pouches. The type of pouch one had could depend on if one made it for himself, had an itinerant cobbler or journeyman make it, or if it was made by a Saddler or other professional leatherworker. However, even if a professional Saddler (or his apprentice/s) made it as "bespoke work" (custom ordered) there were different levels of how it was made, depending on the customer's desires and "the depth of his purse" or finances.
Probably the simplest and least expensive Militia Cartouche Pouch was made from two pieces of leather sewn together to form the pouch. As you can see from the originals, it may or may not have been turned inside out after sewing them together. Then they sewed on another piece of leather for the flap. A leather or metal button was sewn onto the body of the pouch and a slit cut in the flap to keep it closed. Finally, they stitched a leather or woven strap without a buckle to the back side of the pouch. OK, so why did they do it that way?
Now I want to state I don't have historic documentation to back this up, so this is speculation on my part from looking at originals and things I've read about them. For a long time, it didn't make sense to me that they made the flap from a separate piece of leather they had to stitch to the pouch. Why not make the back of the pouch and the flap from a single cut piece of leather, so no need to stitch a flap on? My guess is that they were more concerned about using the entire hide in the most efficient manner, because leather was even more expensive then than it is today.
The pouch didn't need to be made from the best leather on a hide, which is the area of the back and tops of the shoulders. The belly leather is "stretchy" and that's why you don't make belts, straps or anything you want to hold it's shape from the belly leather. However, the pouch itself could be made from belly leather because all it had to do was hold a wooden block and since it is stretchy, that actually helps you to turn it inside out after wetting it. IOW, a more economical use of the belly leather.
Now, you want the flap to hold its shape because it not only secures the block and cartouches, it is also the primary protection against rain and snow. If stretchy belly leather was used for the flap, it could stretch out of shape and not perform those functions adequately in the field. So they deliberately used leather further away from the belly for the flap and thus had to sew that piece on. BTW, did you notice how a fair number of the original Cartouche Pouches had flaps made of much thicker leather than the leather used for the pouches? When done, this was done purposefully for better protection of the Cartouches inside the pouch and not only from the elements, but also from stray sparks from a soldier near you in formation when firing.
When they used a strap without a buckle, they did not make a "one size fits all, even if it fits most people badly." (Actually that did not become common until much later in the 19th century and unfortunately is how too many repro's are made today.) What they did was sew one end of the strap to the back of the pouch, then the maker had the customer hold the pouch in the position he wanted to wear it. Then the maker ran the too long strap over the customer's shoulder and back down to the pouch. The maker marked it for cutting where it held the pouch in the position the customer preferred and then took it off the customer, cut the end of the strap and sewed it onto the back of the pouch. Viola! The strap then correctly fit the customer.
I sort of wandered into this when I first began making 18th century sword belts in the late 1970's. I found the only way to attach the sword frog or sword and bayonet frog was to have the customer put the belt on, then mark the position on where the frog straps had to be sewn on so they would lay correctly on the person. Decades later when I got my copy of Eric Myall's DVD
Part 6. Making a Militia Cartridge Box, I was happy to see I guessed correctly all those years earlier, because that's the way Eric made and mounted the strap.
Still more coming.
Gus