Historical Rules · 16 July 2010
Rule 205.3
Official source text from the 16 July 2010 Rules release
A spell or ability that affects cards from a particular set checks only for that set's expansion symbol. A card reprinted in the core set or another expansion receives that set's expansion symbol. Any reprinted version of the card no longer counts as part of its original set unless it was reprinted with that set's expansion symbol.