Jeweled Amulet

A mana battery with a memory, and the charge-counter clause is the entire design. The artifact's own ability can never add a counter while one is already there, so it works as a single-mana holding cell: you load it on one turn and discharge it on another. The "noted type" wrinkle is where the cleverness lives. The activation cost can be paid with any mana you have, and the amulet records the exact type you actually feed it. Pay with a colored source and it stores that precise color for later, smoothing a hand that has the spell but not the color on the turn it matters. Pay with colorless mana and colorless is what comes back; the amulet is faithful to what you spent, not to what you wish you had spent, so banking a specific color requires producing that color now. The acceleration is real but narrow, and it runs through a timing quirk. Charging taps it and releasing taps it again, so it can never do both in one turn; the payoff always lands a turn after the investment, the way a delayed Lotus Petal behaves, and that delay is the price of the zero-cost frame. The era that printed it was still feeling out how much accumulation a permanent could be trusted with, and the answer here is barely any: as engineering it is tidy and self-limiting; as a card it asks you to decide one turn early which color you will wish you had, and to commit the right source to it now.