There's a lot of room for error in reading the ISMACs lists.
For instance,
http://ismacs.net/singer_sewing_mach...l-numbers.html
G2606143 is a model 27 with serial number allocated in 1913. My model 27.
Some people however will look my serial number up and say it's a 15 with serial number allocated in 1910.
This is NOT the fault of the lists.
When we go through the lists, some of us will stop at the first time we see that G2606 falls within a set of serial numbers. The first time that happens - it's a model 15. Some of us however will notice that the first time the first part of the serial number matches has too few numbers. G followed by 6 numbers but my serial number has a G followed by 7 numbers. Those who saw that will go further down the list and correctly identify the machine as a 27.
I've had automotive partsmen - people who work with long parts numbers all day every day - get these numbers wrong and argue with me.