The Remington Rolling Block Firearms by Konrad F. Schreier, jr book that I have indicates in the intro text, the following: In the chapter on The No.1 Sporting Rifle it states '.....the .45-70 "Black Hills Rifle"', all of which were No.1 Sporting Rifle models introduced in the 1870's.
The advertisement as the OP listed is also present in this chapter and it states 'This Rifle is only made for 45 calibre, 60 grain, center fire, ...
Now this is confusing because the intro could be interpreted as that the rifle handles the standard 45-70 casing. But then the original ad could mean the 45-70 casing should only be filled with 60 grains or that the rifle is chambered for the Remington preprioritory 45 Danish casing which is a 45-60 (see link below). The 45 Danish was created inorder to sell rifles to foreign governments thereby by-passing the ruling of not selling Govts controlled cartridges to enemy governments. Two enemies using the same ammunition.
https://www.loaddata.com/Cartridge/117x51Rmm-Danish-Remington/7095I have a Danish that was re-chambered to 45-70 but the bore is 0.456, the same as the Win 76 45-60 diameter and bullet size.
Without seeing an original 28 round barreled RR Sporter and making measurements I think everything is just a guess.