Historical Rules · 22 April 2021
Rule 103.2b
Official source text from the 22 April 2021 Rules release
If a player wishes to reveal a card with a companion ability that they own from outside the game, they may do so after setting aside their sideboard. A player may reveal no more than one card this way, and they may do so only if their deck fulfills the condition of that card’s companion ability. (See rule 702.139, “Companion.”)