It depends on which place the number is in. If the number is in the ones, tens, hundreds, or potentially even thousands place, it probably won't make any difference in determining the approx. year of manufacture since Mr. Postman provided a range of serial numbers manufactured each year.
For example, sn 204318 would have probably been manufactured the same year as 204818. (Disclaimer - Before anyone breaks out an AIA book and tells me I'm wrong with these two serial numbers, please note I just made this up as an example)