You got it and yes the cartridges are necked. They have a gradual slope which is hard to see in the photo.
The Winchester model 1876 was Winchester's attempt to produce an express or big game repeater. While their 1873 rifle would become known as the "gun that won the west" it was still only a .44 caliber rifle with a cartridge limited to 40 grains of black powder--a round that is considered a pistol caliber cartridge today. The 44 WCF or as it's known today the 44-40 will handle medium game, but not heavy game like Bison, Elk, or Moose. It was not the man stopper that most militaries wanted.
Note: Black powder has over 4 times the bulk of modern smokeless so charges were constrained by the size of the shell casing back in the day. This is not an issue with smokeless powder in modern cartridges.
Winchester wanted to created a repeating rifle that would shoot the new 45-70 government cartridge adopted by the US military for their 1873 single shot trapdoor Springfield. Due to the restrictions of the toggle link action that Winchester used in their rifles, the cartridge sizes were limited unless they increased the length and size of the frame. The 1876 is like an 1873 on steroids and weighs substantially more.
Within practical reason, all Winchester could accomplish was to lengthen the receiver enough to handle a .45-60 cartridge. However, by designing a necked cartridge they were able to also offer a 45-75 to approximate the ballistics of the US gov 45-70. and even a necked 50-95.
Standard rifle barrel lengths were 26 inches. Of course the mounted police would be operating from horse back so they chose a full stocked carbine with a 22 inch barrel. It also included a saddle ring sometimes referred to as a sling ring.
Here's a shot of a regular 1873 rifle shown above an 1876 model so you can sort of see the difference the size and bulk of two rifles.
The mounties started purchasing the 1876 carbines shortly after they premiered and continued until the early 1880's. The Texas Rangers also used the 1876's for a while. Due to their small numbers and hard use, any surviving carbines issued to the mounties or the rangers bring premium prices if they happen to show up at auction.
The 1876 was also favorite rifle of Teddy Roosevelt for North American large game. Winchester only produced a little over 63,800 of the model 1876's. Thanks to a new internal mechanical design by John M. Browning, Winchester introduced a new big game lever action in 1886. The new model 1886 big game repeater had a very strong but still smaller action that essentially made the 1876 obsolete.
Rep given