There are two Baptisms on this site as follows:
Year Surname First Name Fathers Forename Mothers Forename Event Parish/Chapel
1912 MARSHALL Clifford Albert Albert Lilian Baptism Coleford
1912 MARSHALL Gladys May Albert William Lilian Baptism Coleford

Next step is to check GRO Registrations for mothers maiden name:
Name: Mother's Maiden Surname:
MARSHALL, CLIFFORD ALBERT WARD
GRO Reference: 1912 J Quarter in MONMOUTH Volume 11A Page 42

Name: Mother's Maiden Surname:
MARSHALL, GLADYS MAY WARD
GRO Reference: 1908 D Quarter in MONMOUTH Volume 11A Page 24

I think that puts it beyond doubt. Either the birth was registered late or the date of birth quoted in 1939 Register is incorrect - we will probably never know!

Mike