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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@MookyTroubadour
I see these japanese dates frequently stamped on components of japanese synthesizers in my workshop. So I'm somewhat familiar with this. And I'm glad they do stamping parts because sometimes there is no other way to date a synth.
(Roland as a brand is different, they have a date scheme in their serial numbers)