Leg
If you've been to the moon in the past, you had to get the special stamp on your leg.
This leg stamp not only proved you went, but lets everyone around you know you've been to the moon before!
きゃく
あし
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You've only met one other person with the same mark on their leg and it was Kyaku (きゃく) the customer. Kyaku visits many places all over the world, so it only makes sense that they've also been off-world too.
Picture Kyaku lifting up their pant leg and showing you all the stamps from all the different places they've been. There are even some planets on there!