Rawn,
Just curious... Have you contacted the Thomaston Historical Society? Just wondering, because J. Henry Allens studio was located there and I thought maybe they would have other photos taken by him and perhaps more baseball pics??? I would have contacted them, but thought you probably have already gone that route...
I have been trying to research the heck out of this thing, but have come up empty handed. One interesting thing is that I did read an article about the manager or a player (I cannot remember) of the Union team out of Pennsylvania (I think). Rather tragic, But he apparently went to the coast of Maine (not to far from Union, Maine) on vacation and the article stated that while his family and friends were watching him swimming in the ocean he went under a wave and never came back up.
I bring this up, because I am wondering if there is a chance that team in the stereoview is not from Union, Maine but is a "Union Base Ball Team" that coincidentally visited said place?
Potomac Yank,
I appreciate your support with this and would certainly like to meet up with you and other fellow members! Its nice to find people who are into the same things you enjoy. For me, The hunt for the story behind something is most enjoyable. Ironically sometimes I almost wish I didn't solve the mystery to something because then I feel like I am lost or had a kid leave home.

But there is always another mystery just waiting... somewhere!