Pony Racer,
If you can document their existance (in that configuration), go to it....
Bill
PR,
I've been looking in some of the sources I have on hand, and you might want to try to find a copy of R. Stephen Dorsey's book, "Indian War Cartridge Pouches, Boxes and Carbine Boots". On pp. 121-124, there are several "King's Patten" "cartridge boxes". While they weren't accepted by the U.S. Army, they were obviously of "military pattern". The one shown on p. 124 is for 20 ga., probably with the 1881 Trapdoor "forager" in mind, but it wouldn't be hard to make one in 12 ga.
When open, the bottom part folds down in a manner similar to a McKeever, but without a rod hinge, just the continuation of the leather. There are side pieces which are fixed to the side of the fold-down piece, and fold out flat when the box is open. A finial stud is attached to these side pieces, and there are a pair of straps, one on each end of the box, which hook over the studs when the box is folded up closed. A large flap folds over the top of the whole thing, fastening to another finial on the bottom of the closed box.
The thing to keep in mind about "authenticity" when it comes to some of these accoutrements is that a good saddler sergeant could make up about anything, and company commanders were REAL lenient about the use of non-regulation equipent...in the field!
Hoping this is of some interest!