MOD. C3030 may be a house brand made by Marlin for a store chain. Marlin made 336 .30-30s for Sears, Western Auto, etc all under the stores' brand name and model.
ADDED: Marlin made 336 rifles in 30-30 for the Coast to Coast chain stores and every ref. I have found to C3030 Marlin brings up the speculation it was a house brand made by Marlin for Coast to Coast (kinda like the Ace hardware store chain.)