Turns out this date is using the Japanese imperial calendar. The date is written year, month, day.
So this was sent 25 December, in the 5th year of the emperor.
@raspberrypanda might be interested.
@vozoto @raspberrypanda how can one tell the difference between December 25 of the fifth year of the previous emperor?
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@raspberrypanda @MookyTroubadour yeah no idea. Maybe the other characters within the stamp give that info? Though none of them match up with anything here: