According to here it's a "Belgian Cross - recycling symbol commonly seen on alcoholic beverage bottles sold within EU":http://web.onetel.com/~uncletony/recycle.htm
You're not the first to ask this..http://www.flickr.com/photos/ben-woodward/2158542209/I finally tracked down an explanation here: http://www.pandava.com/cms?1104&ecolabels&fr&&"A few years ago, the authorities decided to introduce environmental taxes on packaging pollutants. They hoped to push consumers to choose greener products. There are two symbols: (1) the product is subject to environmental tax, (2) the product is exempt from environmental taxes. In Belgium we do not often meet these symbols because most packaging is eco-tax-exempt if the manufacturer undertakes to recycle a certain amount. There are a symbol of eco-taxes on batteries."That looks like the second variant, which means it's exempt. The numbers will presumably be a registration number for the product.
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