Here is a site that lists what team each player in the set is on.
http://www.krispaulw.com/cgi-bin/set...setlist-01.txt
If you breakdown the set by city, you get:
Boston NL 10
Brooklyn NL 6
Philadelphia NL 6
New York NL 5
Louisville NL 4
Washington NL 4
Chicago NL 3
Cincinnati NL 3
Pittsburgh NL 3
Baltimore NL 2
Cleveland NL 1
St. Louis NL 1
I didn't break out the duplicate issues (Ward, etc) but about 20.8% is from Boston.