Sounds like an excuse to run out and buy another 1905 target to use until you find a screw. Gun Shows, I cruse the gun shows looking for guys that have junk boxes, etc and pick up old rear Smith sights. So far I've picked up 3, 22/32 heavy frame 22 rear sights. Usually for next to nothing because the vendor doesn't know what they go to and they have nothing in it because they bought out an old gun shop for next to nothing when the old boy retired.
The local gun shop has a gunsmith and I dig thru his sight box for any part I need. I picked up a not factory nickel 1948 K 22 that the trigger, hammer and rear sight had been nickeled. Went up to see Bobby and dug thru his box and found a correct blued rear sight. $15.00
SWCA 892