In 2005, Velvac started putting the part # on the bottom of the convex glass housing of their mirrors (for standard mirror heads, the part # is underneath the housing of the mirror head.) Use image below to help identify your part number. If your mirrors were 2004 and older, you'll have to email Jack@pacificrvparts.com so Jack can help you figure out what you need.
How to measure a Velvac 2020 mirror
(Example below shows proper way to measure a Velvac 2020 Mirror Arm)