I had a similar problem, and when I repair things I make detailed notes. I don't have time now to add many details, but my notes are at the end of this post.
I think the part you need is the elevation stud. What appears to be the elevation screw when you look at the sight is actually called the elevation nut. If you do remove the elevation nut, there is a very small spring and plunger that you have to be careful not to lose.
My notes are below. You can find details about the part numbers through google. Hope the formatting is readable. I use note outlining software and it comes out a little funny when I paste it here.
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Repaired rear sight
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1 Summary
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The elevation screw on the rear sight of the model 19 got stripped.
2 Parts
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2.1 Elevation Nut
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Smith & Wesson Rear Sight Elevation Nut S&W All Models
From Midway:
Product #: 257547 Smith & Wesson #: 051020000
2.2 Elevation Stud
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Smith & Wesson Rear Sight Elevation Stud S&W 36, 14 to 19, 24, 25, 27, 28, 29, 57, 66, 67, 325PD, 329PD, 500, 586, 610, 620, 624, 625, 627, 629, 648
From Midway:
Product #: 433927 Smith & Wesson #: 071800000
2.3 All other parts saved
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I was able to re-use all the other parts in the old sight.
3 Bent rear sight body to reduce tension
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The body of the rear sight is really a leaf spring which pushes up. Tension on this spring is what holds the sight up against the elevation screw. On this sight, the upward tension is very strong and I suspect that is why the elevation screw got stripped. I carefully bent the sight to slightly change its shape and reduce the upward tension.