This knob is perfect for that primitive "well-aged" style.
Oil Blackened- Raw iron coated with oil. The oil will wear and expose the iron beneath which will rust. Left as is, this finish will erode, or rub off. This can be prevented by sealing with furniture wax or a clear coat sealant.
This item is handmade. As a result some variations in the size, boring, and shape should be expected. However, all efforts are made by the craftsman to keep the pieces as consistent as possible.