One of these minute details: 481 in a square. Why?

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You think so 😁?

But why do you think there is a inscribed box/square around it which you never see at other examples 📖
 
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What we can say is that all three examples are very close in serial number and that they all seem to have import marks on their balance cocks ( I think. It's hard to read the letters).

If they are import marks - the squares may have something to do with the customs process, though that's a wild guess.
 
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The hand inscribed wording has something to do with the import process but only #1 and 3 have that. Curious about the serial of #2