I am assuming you mean the PC4GR (.40 S&W) carbine (with the ghost ring sight). If so here is the Ruger info: Ghost Ring Sight Base Screw, 2 Req'd G06401 PC9GR, PC4GR
I checked www.gunpartscorp.com and they don't have any and doesn't show any tech data on them.
You might try www.jackfirstgun.com
Sorry.