It's a simple question and should have an easy answer, right?
The simple answer is that pinpointing an exact date for any vintage Singer can be difficult, due to incomplete, conflicting, and contradictory information.
But a bit of detective work should help you date your machine to within a 2-3 year period.
Serial number and model number are necessary to start the process. Armed with your serial number you can visit Singer's website which provides the year the serial number was allocated. Unfortunately, serial number blocks were often hundreds of thousands of numbers. Depending on the factory it could take a few months, a few years, or a decade for all the numbers to be assigned to individual machines.
Relying on the allocation date alone can be misleading.
For example, 401 machines carry an NA or NB serial number. According to the Singer site, all serial numbers beginning with NA were allocated in 1951 and NB serial numbers were allocated in 1956. However, the copyright date on the 401 instruction manual is 1959 and the model was marketed from 1959-1961. This gives us a truer date range for a 401 than the allocation date alone.
On the other hand model 15-91 was marketed from 1933 to 1956 (see chart) and were cranked out in huge numbers in the US and Canada. Since the more prolific factories (such as Elizabethport) burned through serial numbers faster than Anderson, new blocks were allocated more frequently. For this reason, the allocation date for a 15-91 serial number is a truer manufacture date than it is for a 401.
OldSewinGear is the collaborative effort of retired repairman Gary and daughter Barbara. We love old sewing gear and enjoy sharing what we've learned in our vintage sewing machine adventures. We are located in Roseburg, Oregon.