The Veterans' Guard rifles had both barrel and forestock shortened.
A couple of comments from the gunsmithing side of things. IF such a conversion were done as an official experiment, the work would have been done in an ordnance workshop, factory toolroom, etc. While some Mk.II barrels have flat crowns, Mk. III crowns have a radius. I would expect a workshop job to have tool marks from a lathe, whether flat or radiused. The crown described suggests handwork. Similarly, to set the front sight base back on the barrel, either the barrel diameter would need to be reduced, or the sight base's bore would have to be enlarged. The front sight base illustrated seems to have numerous light peening marks. This is a way of enlarging a band. Peen lightly all over the band, tap the base further onto the barrel, repeat until the base is in the desired location. This technique works. It is also the only explanation I can think of for the surface texture of the band. A modification done officially would likely have involved either the barrel being reduced on a lathe, probably while being crowned, or the sight base would have been reamed to fit.
I suspect that the conversion was unofficial. IF the conversion was done by the military or the Ross factory, this would be a very rare rifle, so careful investigation is appropriate.
Just out of curiosity, is the bolt sleeve pinned to prevent incorrect assembly? If it is, the rifle was certainly still in service during WW 2. Will a bayonet fit?
I suspect that if the Mk. III Ross had been manufactured with an overall length similar to the SMLE's, the rifle would have been more popular.