Posted By:
DavidThere are several well known sets, including a variety of 19th century comic trade card issues. There are also lots of obscure though legitimate singles from the 19th and 20th century, including foreign and trade cards. SCD has a physical page limit on it's book, so many obscure or not popularly collected cards and set won't be included. There just isn't enough space, especially with all the new issues. For obscure singles to be included, they really have to depict the big guns, ala Ty Cobb and Babe Ruth.
For an interview with Bob Lemke, where he discusses these and other issues, check out the below link.
http://cycleback.com/lemke.html

1920 Frederick Foto Babe Ruth (from a set of PCL players)

1920s Grover C. Alexander Clothier's Trade Card