Well, it's not technically "extra" metal. Those coins have the same amount of metal in them as any other cent. The difference is that those were struck using dies that had small breaks in them. Part of the steel of the die broke free and came out, thus reducing the depth of the die in that area, allowing the metal from the coin to flow deeper into the die than it should have.
On Lincoln cents, these breaks in the word LIBERTY are somewhat collectible among a group of people who collect what are known as "BIE" cents. They are so named because the break makes it look like an extra letter was stuffed between the B and E of LIBERTY, effectively making the word appear as "LIBIERTY".
They have little premium value to these collectors, and generally no premium value on the general coin market. There used to be a national club for collectors of BIE cents. They even published a guide to BIE cents back in 1972. Interest in these coins has gone down quite a bit since.
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