The brief discussions of "the locking screw" above are, let's say incomplete.
Here's what's what: BOTH screws are employed to adjust and lock the sight---the very first from S&W that won't "shoot loose"---after a mere 50 years or so of making target guns.
Step One: Make the initial elevation adjustment with the aft screw---get close.
Step Two: Make the final elevation adjustment with the screw immediately in front of the aft screw.
Step Three: Turn the aft screw counterclockwise as far as it will go.
You're done! The elevation adjustment is set AND locked.
Here's the why of it. The screw immediately in front of the aft screw is bearing against the frame (top strap). The aft screw is pulling against the frame, by virtue of a flange nut riding in a slot in the frame.
This push/pull arrangement assures you the sight will move when the frame moves-----don't hold your breath waiting for that to happen.
It is my considered opinion this is the best sight S&W ever made---in that it is infinitely adjustable AND won't shoot loose. (It's successor, the so called Micrometer sight (still in use today, is it not?) also will not shoot loose, but is not infinitely adjustable (to the best of my knowledge-----considering I don't know diddly about any S&W product produced in the last sixty some odd years!)
Ralph Tremaine
Back to this infinitely adjustable business: This sight moves the point of impact one inch for each ten yards of range with one FULL turn of a screw. Given 360 degrees in one full turn, one may presume you have each 1/360th of an inch to choose from----more actually, if you want to get even more picky than I am. The so-called Micrometer sight moves the point of impact what, 1/2 inch at 25 yards with each click? That's plenty good enough for paper targets, but if you're into lighting matches, snuffing out candles, driving nails, or murdering Bumblebees, that ain't gonna get the job done!!
And another thing---those tiny little screws: Tiny little screwdrivers are to be had at Lowes in the form of the Kobalt Precision Screwdriver Set. Pretty much any Jeweler's screwdriver of the proper size will work too--aside from the fact the handles require you to hold them with your fingers instead of your hands. Lowes version allows you to use your hands. The part number is 0525844. The cost should be somewhere less than $20---for a set of TEN! That suggests they're junk right off, but I've been using mine going on a looooooong time, and they're still good to go! Note 4 of the 10 tools are for slotted screws. Two of those four fit any/all tiny little screws to be found on ANY pre-war S&W. The rest of them are for use on other stuff.