This particular pouch came from a box full of here, and I just reached in and grabbed one at random. You can see it's already had a rough life, but has held together.
This double, dual-magazine carrying set of pouches is made of a high Denier (800-1000 I'd guess, by the feel) and are built into a single backing. From the frayed stitching marks, I'd guess that they originally came with some sort of Fastex style clips to act as the primary closure points for the box-like pouches, but was probably removed to speed things up in the field.
A broad loop-field panel on the front of the outside of the pouch acts as the primary closure as a result, with a corresponding hook-field inside the box-top lid. A tongue of webbing with a hook-field can be pulled out from inside the pouch to mate with the loop-field on the front to neutralize the hook-and-loop closure entirely, which was probably useful when the original Fastex loops were still in place.
Internally, the pouches have been given a polyurethane coating for additional waterproofing, and are double stitched throughout. Drainage grommets with a wire mesh grit filter are found in the bottom of each, this would act to keep the grit and sand out of your magazines, and was a nice little feature.
The inside of the box-lid has a little pocked sewn in, big enough for a MRE packet of waterproof matches, a P-38 can opener or maybe a button compass, I found a couple of match-packets and even a couple of water purification tablets when I went through the pouches.
The back panel of the pouches have several different ways to mount, and also shows the vintage of the pouches. ALICE style brass-wire belt hooks, and no PALS-MOLLE to be seen. A pair of steel rings allows extra strapping to be attached to sling the pouches, though the angle of the two ring mountings being different suggests there is a "right way" to do this. A question for the veterans out there. What gives?
The pouches have been stamped with a NSN, but they are all so aged that I couldn't make out the cage numbers. I'd say these pouches would work well doing double duty in much the same way as any of the Platatac FUP or bigger pouches I've grown to love, but with a far older look.