The earliest M&W catalogue I have is from 1931, and the 414 is listed, but just with the square-ended blade.
In the 1934 catalogue there is an optional additional narrow graduated blade.
In the 1936 catalogue there is another option – with the blade with angled ends.
If you have either of the optional blades this may give you an earliest date.
I have no clue as to whether the additional number is a date.