Josh,
Thanks for checking!
I think you may have seen another example of the top postcard in Legendary Auctions' November 2010 auction. They had one in Lot 400, which featured 3 Phillies postcards.
Here's the photo from the Legendary site:
To me it looks like the top card that I posted in my original thread is from Spring Training. Different players wearing different uniforms from other teams may have been present either because they were training in the same place (for instance many teams trained in Hot Springs), or because a few new players joined a team and were wearing their old uniforms or sweaters.
By the way, isn't that a Boston Doves uniform not a Brooklyn Dodgers uniform?
You may be right on Luderus and Magee -- I will compare several photos to see.
Thanks again, Josh.
Robert S