My S&W model 27-9 had the same short sight base screw and blast marks on the cylinder. This minor issue is easily corrected.
Order a ten-pack of Forster Slotted Oval .200" Diameter Head Screws 6-48, Blue from MidwayUSA, $6.39.
Forster Slotted Oval .200 Diameter Head Screws 6-48 Blue
Remove the existing S&W rear sight screw. The head of the Forster screw is slightly larger than the S&W screw. Spin the screw in a drill chuck and turn down and shape the head with a small file to match the S&W screw.
Trim the screw length to fit the end flush to the inside surface of the frame top strap. Trimming and trying the length of the screw is easier if the cylinder is removed. Remove the yoke retaining screw with a fitted screwdriver and the cylinder/yoke assembly will slide out.
Polish the screw head with 400 grit sand paper. Heat the screw in an open flame of a propane torch or gas stove a few seconds for it to turn blue. Coat the screw with gun oil and assemble the rear sight. Install the cylinder/yoke assembly. You are done.