@DotardTed
I believe these were originally Russian military district designations that the vehicle belongs to: https://en.wikipedia.org/wiki/Military_districts_of_Russia
Z for Zapad (West in Russian), V for Vostok (East), there was O (Central district?) in use in the beginning, but I don't see that often nowadays.
Then apparently someone decided that sticking to Z would be a good idea and they went with it.
@Mummabear