They were made from 1955-1964. As your rifle shows- Marlin sometimes put decent walnut into their small caliber guns. There were other variants of the same gun- in different calibers- .22 magnum after 1959, .256 Win Mag, and .30 U.S. Carbine cal. The steel reciever has a higher re-sale value than the aluminum version of the 56. Can't help on value- best bet is to watch auctions like gunbroker and auction arms to get an idea of what they go for. Generally, guns are not a good money making investment...
Regards,
Doc Sharptail