Here you go:
BROWNELLS : Schematic for Marlin® Camp Carbine - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools
I'd say part #29 is what you are after - but there is a schematic for verification.
550-000-289
Sight Base Screw, Front
Factory Part Number(s):
320991
$3.00
Been dealing with Numrich's since 1987 - they've always done me right - lots of freebies show up in my orders from time to time.
Never had any problems with them. Usually fast and accurate service.