In fact, it's already packed to the maximum (and thanks to Aeron, he helped me a lot on this!) There are 3 ID taken for 12 different blocks in one...
Blocks ID until 255 are supported by default since MC 1.4 (or 1.3?)
Thanks very much! I've been asked many times for a bukkit integration, but as I don't play in multiplayer, I would be unable to make and test it....
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