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For both Herring Gull and Lesser Black-backed Gull, new black codes were used in 2013 in the Netherlands by Roland-Jan Buijs. This time it is
BLACK TIBIA WITH YELLOW INSCRIPTION. A dot separates the two digits.

It is important to note the yellow inscription, as there have been black tibia rings used in the past, only then with WHITE inscription, and without the dot in between.

And then of course a few examples...

And here are a few example birds with black tibia rings with WHITE inscription. Note that the metal rings in these birds are at the tarsus, while the example birds above with yellow inscription had the metal ring at the other tibia.