I found the Doc Powers Day a couple of years ago at a small postcard show. The dealer knew it was a tough one, as he had several high end cards.
Over the years he's had the Powers card, the Rotograph Chesbro I posted earlier, a 1912 Red Sox team card, a San Francisco Seals card that I was beaten to by the guy who literally got to his table a minute before I did, and this great card:
This is the 1910 pennant winning Roanoke Tigers of the Virginia League. Many of these players were featured on T209 Contentnea and T206 cards.
The card was sent by a player, presumably shortstop Eugene Newton. The message on the reverse mentions that the team barely topped the league and was postmarked in September of 1910.