Here are some data and trivia about yen coins, though I don't think of a good answer to your first question.
I love how the 5 yen coins have the hole in the center and how they look. Do you know the concept of graphic design for the 5-yen coin?
Each of the four graphic figures (three on one side, one pair on the other side) is representing something.
Maybe all 5-yen coins you are going to get are the current version with characters in a Gothic font.
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http://www.boj.or.jp/note_tfjgs/note/valid/img/coin_5f_03.jpg-
http://www.boj.or.jp/note_tfjgs/note/valid/img/coin_5b_03.jpgThe older version ( -1958) has characters in a neat handwriting font;
and, among them a kanji character meaning a country is in an old shape.
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http://www.boj.or.jp/note_tfjgs/note/valid/past_issue/img/coin_5f_02.j...-
http://www.boj.or.jp/note_tfjgs/note/valid/past_issue/img/coin_5b_02.j...The both versions of 5-yen coins are 3.75 g in mass, which is equal to
monme (abbreviated as
me) and one-thousandth of
kan in the old Japanese
shakkan measurement system.
As to 10-yen coins, the difference between the older and the current versions is on the edge.
The current version has a smooth edge, while the older version ( -1958) has a serrated edge.
I have two older-version 10-yen coins in my wallet, though their edges have got smoother.
There have been two versions also of 500-yen coins, but they are practically different types.
They differ in composition, mass and graphic design.
The current type has a slantwise-serrated edge and a surface with a latent image.
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http://www.boj.or.jp/note_tfjgs/note/valid/img/coin_500_02.jpgThese features are meant to prevent fake coins,
and it seems that the older type of 500-yen coins are being withdrawn for the same purpose.
Now in these days, all 500-yen coins I get from vending machines are the current version.
Japanese yen coins
: 1-yen coin, 1.0 g, 20.0 mm, 1955 -, aluminum;
: 5-yen coin, 3.75 g, 22.0 mm, 1959 -, brass
/ (older ver.), 3.75 g, 22.0 mm, 1949 -1958, brass;
: 10-yen coin, 4.5 g, 23.5 mm, 1959 -, bronze
/ (older ver.), 4.5 g, 23.5 mm, 1951 -1958, bronze;
: 50-yen coin, 4.0 g, 21.0 mm, 1967 -, cupronickel;
: 100-yen coin, 4.8 g, 22.6 mm, 1967 -, cupronickel;
: 500-yen coin, 7.0 g, 26.5 mm, 2000 -, brass-nickel
/ (older ver.), 7.2 g, 26.5 mm, 1982 -1999, cupronickel.