I'm a bit confused, I can't find this item referenced in any other source. Why does its reference say "Basic Rules" if it doesn't come from there?
Take a look at the monster Shield Guardian which is in the Basic Rules - the monster has, as part of the description, an item called Master's Amulet that allows the wearer to control that shield guardian. As such, it was deemed to be an item that is part of the Basic Rules (even though it doesn't appear in the item listing).
Thanks for the reply! How was the rarity determined out of curiosity (no rarity is listed in the shield guardian entry)? It's interesting because this item (and it's rarity) is a matter of some relevance for DDAL players. Was that something that came from WotC?