http://ismacs.net/singer_sewing_mach...-database.html has a database for Singer Serial numbers. From my understanding, the dates given for the serial number is the date that the serial. number was assigned and would have been manufactured sometime after that and maybe even shortly after the date the next numbers were assigned for that particular model. I know that others have found some mistakes in the database. Sometimes it is a misreading of the number either on the machine or in the database.
I've seen ads for machines listed as being for a model number because they had a manual for that model number, however the machine shown is not the machine that the manual is for. Evidently, the person posting didn't compare the picture in the manual to the machine they have sitting in front of them.
What parts are you looking for? The domestic model number 15s take standard low shank presser feet. Parts can be found on ebay, craigslist, thrift stores and online stores like <cite class="iUh30">https://shop.sew-classic.com</cite><cite> </cite>
One of the best threads about worth of vintage sewing machines is
"How Much Is My Machine Worth?" Question Answered
Bottom line value is what price seller and buyer agree on. I've seen a couple of machines on craigslist that keep getting listed for years on end. Evidently they don't get the message that their machine is not worth what they are asking for it and unwilling to lower the price.
Treadles are fun to sew on and are good straight stitch machines.
Janey - Neat people never make the exciting discoveries I do.