I'm no expert on these boats or N&W, but I do not believe they had specific hull numbers - (IE a sequential number based on the manufacturers building) - rather they have a HIN number. A HIN (Hull Identification Number) number was made mandatory in 1973 (technically November of 1972 – and some builders were using the format since the 1960’s) by the coast guard and is more like a registration number. This site shows you how you can read yours.
http://www.boatsafe.com/nauticalknowhow/hin.htmNow if your boat is older than 1973 and has no HIN your best bet would be to figure out through previous State registrations the probable build date (year is prob best you can do).
Note - the HIN # will tell you the builder and the date of build but NOT the sequential build (IE "is this the 3rd Herreshoff America built?").
Lastly if your hull DOES have a Builders Hull Number it would be found on a builder’s plate - usually a small metal plate stating the Manufacturers name. They are usually fairly easy to find. Here’s what mine looks like on my Marshall 22:
