1940 will be a tough year to find in a Featherweight. Because of the war there were fewer being manufactured. Singer issued a "lot" of serial numbers to a model #on a specific date and that lot was used until that many machines went through to use up the #s, then a new "lot" was issued. The serial numbers issued in 1940 range from AF477181 - AF596880, only about 45,000 total (although ones with numbers AF371856 -AF391855 MAY be 1940 manufacture since that lot of numbers was issued 12/5/1939 and included the 20,000 manufactured between then and April 10, 1940 when the lot beginning AF477181 was issued).
I bought both of my featherweights on craigslist (one is 1948 and the other is 1950) but I did not care about date, just getting a good deal. Because you want a specific year of manufacture, you may have to look for a longer time and pay a bit more. Graham Forsdyke in the UK is a very well known Featherweight dealer, his are always completely gone through but pricier. He will look for a specific manufacture date for you. His website is:
http://www.singer-featherweight.com/...birthdate.html