The Metals Desk Riddle

Priya and Marcus are junior analysts on a metals desk. Dana, the desk's star trader, just closed a very profitable position in a single metal at a single lot size, and teases them with a list of ten candidate positions: Silver: $15$, $16$, or $19$ lots. Nickel: $17$ or $18$ lots. Cobalt: $14$ or $16$ lots. Tin: $14$, $15$, or $17$ lots. She then whispers only the metal to Priya and only the lot size to Marcus, and the two talk it out: Priya says, "I can't tell which position it was — and I'm certain you can't either." Marcus replies, "I couldn't at first, but now I can." Priya then says, "Then now I know it too." Which metal did Dana trade, and in how many lots?