The Merwin-Hulbert revolver was not chambered for the .44 S&W but for the .44 MH cartridge, it was only incicnetal that the S&W round would chamber and fire in the M&H guns. Though called "Army" it was never purchased by the Government for military acceptance. It was used by the US Post Office, though. The P.O. guns were .44-40 caliber though.
It is interesting to note that the Merwin Hulbert guns were marked for the ".44-73" cartridge, referring to the .44 W.C.F. Model 1873.
Here's some .44s from Frankford Arsenal:
As a matter of note, the .44 Remington and .44 Colt cartridges are not readily interchangable. There is the mistaken belief that the M1875 Remington could take either cartridge, since Army packets were marked "For Colt and Remington Revolvers." This alluded to the cartridge conversions of the Remington Model 1858 cap-and-ball revolvers. The .44 Remington cartridge was not introduced until 1875 or so.