salvadore is correct. The 45-75 is a necked cartridge, shorter but fatter than the 45-70. It was introduced in the Winchester 1876, the Centennial model, a rifle based on the 1873 action slightly lengthened.
45-75 case is based on 50-70 brass, and is about 1/4" shorter than the 45-70. A load for the 45-70 would be a good starting point.