Historical Rules · 30 September 2011
Rule 206.3
Official source text from the 30 September 2011 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.