Millett Series 100 Adjustable Rear Sight. Box says for S&W N frame. Seems to fit K, L and N. White Outline with .410 blade
These found in my cleanup. ANIB or NIB, I am not sure. I have had them for several years. Purchased for an N frame project that never happened. They appear new. No marks. In original boxes with papers. 2 sights for $60 shipped, or one for $35.