If you look at the picture in #4 you will see a lid in the middle of the butt plate. Small items or extra ammo could be stored there. The Spencer was the most common stock loading rifle of the day, all 1873’s were side load into the under barrel magazine tube.
The cartridge is likely a .44-40 (44 W.C.F), as the 1873 was not chambered (originally) for the .45 Colt.
Good answer on the cartridge by Tijeras_Slim.
Although it was unloaded, we retrieved a UMC .44 WCF cartridge in the stock, dating between 1887 and 1911.