"...production on the 1903A3 began in 1943..." Started in 1942 with some in 1941. Like Steve says, your's is a December of '42.
Rummage around here. Clicking on 'Manufacture Dates' can make your head explode though. Lotta work put in there.
http://m1903.com/
"...a field replacement..." More likely to have been done at some time since 1942. Any rebuild marks on it?