Historical Rules · 17 April 2020
Rule 103.1b
Official source text from the 17 April 2020 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 may do so only if their deck fulfills the condition of that card’s companion ability. (See rule 702.138, “Companion.”)