I think OGC and tiriaq have made some excellent points. The front sight and length do indicate it is a Mk II barrel. The 1905R was available with 22", 24" and 26" barrels but not 28". However, with Sir Charles, one should never say never. The stock and butt plate are 1905R, as is the barrel band. As with this one, the 1905R does not have a magazine cutoff, although that could have been removed. It could be a re-barreled 1905R, or it could be a 1905R that has had the barrel and receiver replaced with those of a Mk II. Since the receivers are identical, about the only way to determine which it is would likely be to look for military acceptance stamps. But, then again, the Mk II was also available commercially, so there is likely no definitive answer. It is also not beyond the realm of possibility it left the factory with that barrel.
And now to that rear sight: The Mk II rear sight base for the Enfield and related sights, which include the Canada Tool, mounted to a band or saddle on the barrel. At least, that is how it is mounted on the Mk II 5*. This rifle does not have that band, so it begs the question - how would that sight base be mounted to the barrel. The correct rear sight for the 1905R is the same as that found on the Model 94 Winchester of the same period.
One final point: Plate 70A on page 142 of The Ross Rifle Story shows a 1905R (with 26" barrel) chambered in 35 Winchester. Is there any possibility this rifle is chambered in 35 Winchester rather than 35-303?