Hi Charles,
I don't think the exact dates of printing for each series has been proven
which could possibly be the reason PSA doesn't put the year on them. For
instance estimates for the 350 series are from late 1909 to early 1911.
As Donnie said you can find info on that in scot's Inside t206 article.
There is also some detailed info about the series and print groups on T206resource.com
It starts with print group 1 and you can continue through print group 5
by clicking on the next button.
http://t206resource.com/Print%20Group%201.html