1/2 PRICE -Pier Movable Sidewalk Ticket at the World's Columbian Exposition

One of three different colors; I don't know if there is significance to the colors--Green, Blue and Red, and I've never done the research to determine 1) Significance of the different letters/series and 2) If the only series are A-to-D, shown here. One might assume that there is a 4th color since the letter series go up to at least D.

If you have moe information on these questions, by all means please let me know.