Jim Kenner and I developed the ones Civil War Leather is selling. In fact, I still carry the prototype. Larry Romano's boxes are made to the original patent drawings, Jim's are made after the production version. The difference is in the top hinge and latch. Jim and I spent a lot of time in the Arizona Historical Society basement measuring and photographing originals for all of his reproductions. He is a real craftsman.
The Blakslee is a pain on horseback! You have to get it on just right or it winds up getting between your behind and the back of the saddle (assuming you are riding a McClellan). Not comforatable, especially at a trot or canter! It's almost impossible to get the strap adjusted so that the box is supported when mounted, and not too tight when dismounted. I can imagine that a lot of Blakslees got strapped to the cantle right next to the saber that the trooper was also supposed to be wearing.
Dismounted it works very well. At one N-SSA national, shooting a '60 rifle, I fired 8 rounds, got 6 hits, had reloaded from the Blakslee and was drawing aim on the last target when someone else got it. All in 45 seconds!