This screw attaches the frame rail to the trigger frame on a Sterling pump. 
 
On newer Arrow models, this is an M6 x 12mm flat head with a reduced head diameter to avoid issues on the aluminum rails. If this is lost, or an aluminum rail is installed on a Classic Sterling, a standard screw is too wide to fit. 
 
This is a brass M6 x 12 flat head socket cap screw machined to a 3/8" nominal head diameter - allowing it to fit in either a stock Arrow rail or one of our aluminum extended rails.