Congrats on your first post!
The stock S&W adjustable rear sight will not fit in the 5906 fixed sight dovetail.
If your 5906 has the later and vastly more common Novak style rear sight, a sight company named LPA cataloged an adjustable rear sight that will fit that dovetail.
It will also work with the stock 5906 front sight.
If you are looking for the old "big ear" S&W adjustable rear sight, you are probably not likely to find a complete sight assembly (except on a used slide) and will need to scrounge bits and pieces on GB, e-bay, Numrich, Midway, Brownells, etc.
John
