 Many varieties of the overprinting of "PANAMA" and "CANAL ZONE" exist, including doubled overprinting, complete inverts and stamps with the letter "A" inverted or completely missing.The coloring of the overprinting varies from violet to bluish-violet, with the overprinting measuring 18 mm.The first comprehensive study of these issues was conducted by George L. Toppan, and his classification of the various varieties that occur in these issues is the most feasible one yet presented.