Dig, you threw me off a little when you said boxlock and listed the serial numbers on your newly acquired L.C. Smith. It is a sidelock and your gun was made in 1947. Marlin took them over in 1945 and didn't start producing the "S" series until 1946.
About the Deeley forend latch, I don't think that is original on this year gun. Of couse it could have been special ordered. David

David