These plates are used on off-road recreational vehicles.
I believe this type started in the 1970s with motorcycle-sized blue on white plates and an A prefix, working their way through B, etc.
All-Terrain new dealer plates are also issued, with in an X1 123 format.
Click the photo to see this type in use.
See Also: Dealer All-Terrain
You can get a sense of how few of these were issued every year by comparing the numbers of the 2000 plate vs. the 2001
When the graphic base was started, “All-Terrain” lost its hyphen and became “All Terrain”. These plates began with the “G” series, and four numbers are now used after the letter prefix.