You are correct in that they aren't the dates they were made. If you look for the next allotment for the model, it's safe to say they were produced some time between those two dates, with a possible overlap of the second date, especially if you have a number that is very close to the end of that allotment. Some of the more common models have allotments happening pretty often, and some others rarely.
I agree with Cari - if Singer allotted 200,000, they probably didn't do another allotment until they had used up all of those numbers. There would be no reason for them to allot more number if they had not used of the first group. With the exception if it was the last group of numbers allotted for a particular model.