OWD,

Have a look at the neat little aperture sight on my Westley Richards .303 double:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=122311&an=0&page=3#Post122311

Of course, it is original to the rifle and not an add-on, but it gives you an idea of how they did it in 1901.

Curl