If you are old enough, you might remember seeing these things in the high school print shop. These large, flat wooden boxes contained lots of little compartments to store metal type used in printing presses. The most common layout was called the California Job Case.
I've been in the printing business since the early 70s. Of course, I didn't actually use metal type in the printing business, although metal type was still being used in some places for special purposes. Back then, the California Job Case was relatively recent technology.
By the way, in high school, we had to memorize the location of all the type! And more than one student spent an afternoon resorting type after knocking over a case!
Although this license plate would have been meaningful to more people back then (someday soon NOBODY will remember it), I figured that it was appropriate for an old printer to put it on an old car.
If you DO happen to be familiar with the job case, drop me a note - I won't feel so alone.
And if you'd like to know a little more about the printing business, just ask - you won't be able to shut me up. (Old guys love to tell stories.)
If you are in to older printing equipment, check out Alan Runfeldt's great printing museum and web site: