The older Hawken sight had the same screw. Dixie lists the sights, but I don't know if they sell the screw seperate. Numrich, may have it, they used to list it.
Just some other options.
The owner's manual is available at Thompson Center. The stocks were proned to cracking, at the lock screw. 35 grains is Max. charge ( for 45 cal.), but I never load more than 25 grs. for fear of cracking the stock. Never place the butt on a solid surface when loading, as many people believe,that is another reason why the stocks get cracked.
They are a great pistol, and very accurate.