I expect your interest here will lead you to what is sometimes called Fixed Sight Targets, guns that had their sights upgraded for target shooting. Or possibly, if you continue down the rabbit hole of upgraded fixed sights you will find you like some of the other upgrades for duty weapons.
Most of the standard factory fixed sights, particularly from before the war, are very poor. A mere groove in the rear and a blade in front. They are often thin and difficult to see for all but the best eyes. Because of this, anyone who needed a fixed sight gun but who had either old eyes or had become accustomed to good target sights would have custom work done to the gun.
A number of gunsmiths would do such work, but these days we are most familiar with the work done by D.W. King out of San Francisco, and due to the wonderful work the Smith & Wesson Historical Foundation is doing; the S&W Service department itself.
I share your interest in fixed sights, or rather I find I became more interested in them because I am fascinated with modified guns and many that I have been fortunate enough to acquire have fixed sights.
Here are a couple examples of upgraded fixed sights done by the Service Department at S&W:
Half Target M&P, Update: Another Police Gun!
Increasingly more Interesting Triple Lock *update*
And here are some guns modified by D.W. King:
Model of 1905 Fixed Sight Target *Update - SFPD Gun!*
Another King Super-Police! (Post yours!)
Personally, I find this particular sight modification just to be fantastic, I can't really praise it enough, the sight picture is just so glorious. Especially considering the age.