Nepperhans are a mysterious pistol.
1857 was a pivotal year for American revolver makers.
In 1850 Sam Colt's revolver patent was expiring, and her aggressively sought to get it renewed. He was semi-successful, and got his patent extended for seven years. Colt returned in 1857 for another extension, but did not get it. So,, in 1858, the revolver market opened up to "legal" competitors who previously struggled to overcome Colt.
Things were particularly good in 1858, as Colt his monopoly, and a craze for revolvers hit the firearms market.
Only one competitor had challenged Cot, back in 1851, but lost, what would be formed in 1855 as the Manhattan Firearms Manufacturing Company of New York. They were well positioned to ride the wave of revolver demand, and when the Civil War broke out.. while Colt and then Remington pretty much had the lion's share of government military contract work... Manhattan became a major supplier of the civilian market (especially so in the "pocket" revolver business).
In 1857 Thomas Bacon (superintendent of manufacturing for Manhattan) , was involved in setting up a revolver factory in Norwich, Connecticut, was sued by Manhattan in 1858. In 1859, in exchange for a $50 payment from Manhattan to settle up debts, Bacon agreed NOT to "... engage, or be in any was concerned, in the manufacture or sale, or in any other way connected with dealing in firearms of any kind whatever, during twenty years from this date..." except for making single shot pistol barrels for Manhattan.
Bacon had had his own company 'Bacon & Co.' which made single shot and pepperbox pistols starting in 1852. And, despite or in spite of the Manhattan lawsuit settlement and and agreement, in 1858 Bacon formed the "Bacon Manufacturing Company" for the manufacture and sale of firearms.
Bacon had a falling out with his company, and was kicked out and/or otherwise left in 1864 and left to form the 'Bacon Arms Company."
He did better, as this company lasted until 1888.
What ties Manhattan, Bacon, and Nepperhan together is the Manhattan removable side plate design revolver(s).
Nepperhan, had a factory in Yonkers, NY. and made somewhere around 4500-5000 Manhattan type/clone .31 revolvers sometime after 1860.
Nepperhan may have produced more of these revolvers than Manhattan did, as Manhattan switched their emphasis away from .31 pocket pistols over to .36 navy types.