Whenever Scrap Trawler or another artifact you control is put into a graveyard from the battlefield, return to your hand target artifact card in your graveyard with lesser converted mana cost.
2/9/2017 If Scrap Trawler and another artifact you control are put into a graveyard at the same time, Scrap Trawler’s ability triggers for each of them.
2/9/2017 The target artifact card must have a lesser converted mana cost than the artifact that caused Scrap Trawler’s ability to trigger by being put into a graveyard. Use the artifact’s converted mana cost as it last existed on the battlefield to determine what may be returned.
2/9/2017 While on the battlefield or in a graveyard, in an object’s mana cost is 0.
2/9/2017 If an artifact is a copy of another artifact with greater converted mana cost, such as Sculpting Steel copying an artifact with converted mana cost 4, Scrap Trawler’s ability can target that artifact card in your graveyard when that artifact is put into your graveyard.