Not a top break, unusual twist, pull forward design. Shape of the sideplate is the giveaway to the make. Look close at the rear of the cylinder, top, you can see where gun rotates to open.
History: Joseph Merwin was involved with marketing revolvers as early as 1856 and first formed an arms company known as Merwin & Bray. This company was folded by 1874. In 1876 Merwin formed a partnership with the William and Milan Hulbert, who owned 50% interest in Hopkins & Allen. After Merwin's death in 1888, the company became known as "Hulbert Brothers & Company". Hulbert Brothers declared bankruptcy in 1894 and in 1896 it was liquidated. Hopkins & Allen continued to manufacture Merwin Hulbert marked guns until 1916 when it went bankrupt and was bought the following year by Marlin Firearms.