Your front sight is not adjustable, do I suppose it is fixed, but adjustable front sights are only seen on rifles, as far as I know. I’ve never seen one on a revolver, although I suppose it’s possible. But, also, your revolver has the front blade that is pinned in place, and it can be “adjusted” in that the blade could be removed and replaced with another, if desired, unlike the non target model variations which contain a blade integral with the barrel.
I have seen a few others like yours with the atypical rear sight. I believe it to be factory, as the half Target Model revolvers are unusual (fixed front and Target rear sight or pinned front and standard rear sight). The others I’ve seen like yours I believe are, or had been, in Europe or the United Kingdom, so I believe this was for competition at Bisley. Again, I believe it’s factory work for Bisley, but it’s certainly possible these were ordered as a half Target Model and the millwork and installation of the rear sight was installed at or near Bisley.