 Few varieties are known in the Mercury dime series. The 1942 / 41 is generally termed an overdate; it is actually a doubled-die error — the obverse die from which the coins were struck took one impression from a 1942-dated hub and one from a 1941-dated hub (until the 1990s, dies required two strikes from a hub for the design to be fully impressed). Sinnock stated that the pieces were most likely struck in late 1941, when preparation of the 1942 dies was under way. Also produced at that time, though less apparent to the naked eye, was the 1942 / 1-D. Another popular variety is the 1945-S "Micro S", with a smaller-than-normal mintmark. This variety was caused by the Mint's wartime use of a puncheon (used to impress mintmarks on dies and hubs) which had been made for use with early 20th century Philippine coinage struck at San Francisco, which had only a small space for the mintmark. Beginning in 1928, coin albums were issued by private publishers, mostly in folder form, which were widely used to collect the pieces. This led to a great increase in interest in collecting current coinage by date and mintmark.
