The date on ISMACS site is the date the number range was allocated, so if you look at that and the next one for your model number, you can get a pretty good idea of when it was made. And you could assume if you have an early number in the range, it would be closer to the allocation date, and vise versa for an other one.
I looked up the 2-19-47 range, and the next allocation of numbers for model 221 is 4/22/1947. Some models that didn't have much production may not get numbers allocated near as often. The tables are rather interesting to study, and it would be really neat to know if Singer had multiple lines to produce different models at the same time, or needed to switch out and do individual productions runs. I suspect they probably had multiple lines going at the same time.