If you're missing the screw, you're probably missing the stirrup too. It snaps over the grip pins and has a nut on the bottom. Phone Hogue, ask nice and they'll probably send you a set.
I bought a used Hogue rubber grip on eBay once that came without the screw. Hogue sent me a new screw and stirrup free of charge.
I've bought several grips as a "thank you" since.
Not E-mail . . . Call themThanks. I didn't have a clue. I sent an email to Hogue about getting replacement parts. Now
to wait to see if I get a reply.