There are three S. Hawken rifles extant that have pistol grips and tang sights...I think they are all in the Jim Gordon collection. Below is a photo taken from his book, as well as pictures of one I made to replicate the idea.
I included a rear sight on the barrel, and wehn I finished the rifle, I sighted it in using the barrel mounted semi-buckhorn rear sight. Then when I installed the tang sight, it was a simple matter to adjust it to align with the rear sight. In function, it clarified the sight picture immensely. The sight I copied was adjustable for both elevation and windage, but there was no calibration for repeatable long distance shooting.
The staff on the tang sight is threaded 1/4" x 80 tpi. The windage screw is #6 x 40.