It'll be a prefabricated stainless steel crown, not made in a lab. Very often used for baby (primary) teeth with large cavities, and placed in a single appointment. More dentists should use them as they have a very good success rate compared to a filling, but takes a bit of getting used to the technique. For girls I call them "glitter" or "princess" teeth; for boys they're "robot" teeth. The number on the outside is to help the dentist know which tooth it usually fits. I usually just polish it off before cementing it. Once, my assistant dropped the whole tray on the ground and had to re-sort them - thank goodness for numbering.