Connected pairs; intentionally darkened.

[villa66]

Two coins connected by the special treatment—and the reason for it—that was a part of their manufacture. This bronze 1903 British farthing was artificially darkened at the mint so it wouldn’t be mistaken for the gold half-sovereign coin that was then circulating in the United Kingdom...

[villa66]

...and this bronze 1925 Dutch 1-cent was artificially darkened at the mint so it wouldn’t be mistaken for the gold 5-gulden coin that was then circulating in the Netherlands.

v.