I really don't know. I have seen many old shotguns, dating to the post Civil War period, when we were still making MLers as Breechloaders were coming on the scene, that have the Hairlip cuts in the skirts. Some of the old guns might date back to the 1840s, from comments made by the owners, or sellers.
The old percussion rifles I have seen in collections also have that hairlip in the skirt, so I believe it was very common and done soon after the percussion system became available. I see this kind of thing done in the early Colt Revolvers, too, and Colt Got his first Patent on his "Paterson Revolver" in October, 1836.
I am going to have to check my original pepperbox "pistols", to see if the hairlip appears in those guns, some of which pre-date the Colt Revolvers.
Just checked the pepperboxes, and my reference books. None of them show a hairlip being used, and many of these guns were being made at the same time that Colt was struggling to get his revolvers accepted.
Allen and Thurber were not attempting to compete with COLT( who did not market short barreled revolvers at first) as much as to provide a cheap, small, multiple-shot pocket pistol. The 3,4,5 & 6 chamber pepperboxes were the most common, and barrel lengths were fairly short. They competed with the single shot pistols, like the Deringer use to Kill President Lincoln, rather than the long-barrel revolvers, in both price, and by providing more than one shot by pulling the trigger. These guns took a lot of time to LOAD, so that having to remove a stuck cap from a nipple was simply one more chore done to load the gun, or "reload" it.
Paul