Single "R" prefix
Replacement always used weight/type code in conjunction. Some long-time collectors contend that "R" was used for government plates. If I confirm this, I will move these to the Government section.
U, V, W prefix
One long-time collector says that "U" was used on School Buses exclusively. My research has pointed to V and W were used on Government vehicles; I'm still trying to separate if certain codes were used on certain types of vehicles (government bus, truck, trailer, etc).
strange prefixes that don't conform
1954 - 2-letter prefixes did not begin until 1964. Personalized did not begin until 1972. This must be a souvenir/personalized with someone's initials.
1963 - 2-letter prefixes did not begin until 1964. Personalized did not begin until 1972.
1964 - Sample plates were always in format A-00-000; is this a prototype/color test?
1965 - Sample paltes were always in format A-00-000 or SAMPLE; unknown what FB prefix stands for
1966 - Motorcycle plates started with letter Z. I've never seen another with Z not in the first position; believed to be overflow after Z9999. In 1967, format changed to allow 5 digits after leading Z.
1989 - Possible that this is a board/commission plate; all 1989 personalized had embossed "89" in lower right corner.