Same story when I looked up the serial number of my 401-A Slant-O-Matic. Singer's site showed all NA serial numbers as 1951.
Another site said NA was used 1951-1959. Charlee, on this board and who's knowledgeable about vintage machines, confirmed what I'd thought, which was that 401's were 1957 and 1958 manufactures. The manual with my 401-A shows Copyright 1957. "Does not compute" that Singer wouldn't have more detailed information.