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When was it customary to drop the period (.) after a date, as in the Scott #649 and #650 examples scanned above?
The 2c Olympic Winter Games issue, released January 25, 1932 (Scott 716) doesn't have the period so it likely ended somewhere in the 3-years between the two issues.
The 1934 Byrd souvenir sheet (Scott 735) looks like the next U.S. stamp printed with a Month, Day, Year format and it also is void of the period.