I am very happy to be able to post again in one of my favorite threads! I just acquired the H.M. Taylor "Tyrus Cobb at Bat" postcard shown below. As Kevin stated above, his outstanding research has turned up the earliest postmark on any H.M. Taylor to be October 10, 1907. As you can see, my Cobb is postmarked October 12, 1907 at 7:30 pm in Detroit. Earlier that afternoon, the Tigers lost to the Cubs in Game 5 of the World Series by the score of 2-0, giving the Cubs the "sweep" by winning four games and tying one. Game 5 was only the second game played in Detroit in the series.
It has been generally accepted that the H.M. Taylor series was issued/sold from 1907 - 1909. The postmark clearly dates this one to 1907. But also look at the different font of "Post Card" and the H.M. Taylor notation on the bottom, which does not include the words "Rights reserved by" as most I have seen do. In fact, the only other Cobb I can find with the same back is on the tremendous vintageball.com site (it is not postally used).
There are a few different backs known for H.M. Taylor PCs, but this leads me to wonder about the correlation between the years of issue and the printing on the back. My hypothesis is that the back shown below, postmarked October 1907 here, was the first style used in the early printing of the H.M. Taylor postcards. Could it be that only ones with this back should be considered one of Cobb's "rookie" postcards? Are postcard collectors now going to collect backs like T206 collectors?
I would love to read your thoughts, comments, and further research!