We occasionally find license plates on sites and they can be quite useful 
time markers since, at least in Utah, it wasn't until the 1970s that stickers 
began to be used on plates.  Before that, plates were replaced every year or 
every few years.  Early to mid 20th Century, collectors were not what they were 
today, so its likely that these were thrown away fairly soon after replacement 
and can often help in dating sites.

Go to the web site: www.worldlicenceplates.com

For some good graphic information on license plates.

Mike Polk
Sagebrush Consultants, L.L.C.
Ogden, Utah