#646192 shipped March 20, 1934.

#659434 shipped January 28, 1937.

Best guess on your rear sight blade is it's a King product (available from S&W on order). If it is, it will almost certainly say so---on the lower portion of the blade. (Back out the windage adjusting screws (one on each side of what I'll call the blade carrier), and lift the blade out. If it doesn't come out with relative ease, back the screws out further. (You can remove them completely with no harm, no foul; but they're quite small, easily lost, and they ain't making anymore of them.) Working over a folded several times Turkish towel makes for a nice (no bounce) surface.

Ralph Tremaine