In 2005, Velvac started putting the part # on the bottom of the convex glass housing of their mirrors. 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 2020 mirror
(Example to the right shows proper way to measure a Velvac 2020 Mirror Arm Length)
Telling the difference between Ramco and Velvac.
The Velvac Standard Head is the only mirror head that Velvac makes with a center post. Here is an example of the standard head. If your mirror has a center post and does not look like the standard mirror head above, you most likely have Ramco mirrors.