Being this "carbine" is a cutdown rifle, the presence of a M1873 rifle rear sight is not surprising. HOWEVER, something you might want to consider before replacing the R sight is that quite a number of original M1873 carbines had their C rear sights replaced by R rear sights. While there is no official documentation that I've been able to uncover in years of research, there is evidence that a number of cavalry company commanders (Capt. Albert Barnitz, of the 7th Cav) requested issuance of the .45-70-405 rifle ammo instead of the .45-55-405 rounds, with a suitable change to the sights! I have examined quite a number of original M1873 carbines that had R rear sights. Now, originally, these carbines, with serial numbers 50,000 and lower, were issued with the long wrist, short comb stock. The stocks were fairly fragile, and when they broke, most had the broken stock replaced with the later M1877 thick-wrist, long comb stock. Of those I examined, the ones that had their stocks replaced in the field or in 1879 at the arsenal, almost all had Rifle rear sights, except in the case where a Carbine rear sight...which didn't match the original finish of the barrel! Of the M1873 carbines that retained their long wrist stocks, most also kept their carbine rear sights. The implication there was that those pieces had left Army control (having been stolen by deserting troopers) before they could have either the stocks or the rear sights replaced.
Whether the use of the M1873 Rifle sights actually compensated for the use of the rifle ammo or not is another question. Since target practice was not universally believed in until the 1880's, most troops probably couldn't tell the difference, and the problem was most likely exacerbated by the increased recoil induced by the full-power ammo.
Just saying...