Looks like somebody installed S&W revolver sights on it, you should be able to replace it with same.
If it does not match the S&W sight you might be able to cast the hole and file another sight to fit it.It really sucks because it is a Randall and I have no experience with them other than I know they are an expensive collectors piece and original parts are really rare. Just my opinion but wouldn't it be better to machine the site to fit instead of the slide?