That's probably the most difficult two varieties of the Four Flags stamps to tell apart. The surest way is to realize that the ATM18 is on very thin paper as compared to the CB20 variety. If that doesn't help to tell them apart, it has been suggested on another thread that the ATM18 has a bit of a glossier look; the CB20 has more of a matte finish.
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Posted 08/07/2013 11:04 pm
