Where is number one on cap for 390?

To find the number one on a cap for 390, you would typically look at the cap's markings or labels. If it's a numerical cap indicating size or a specific measurement, "number one" could refer to the smallest size or a specific designation within a range. Without additional context about the type of cap or its purpose, it's challenging to provide a precise location for "number one." Please provide more details for a more accurate answer.