USA registration number question

[Nick Bombini]

I am curious, what does the W in front of some registration numbers on the trucks mean? I know that an S behind the number means the truck has been suppressed but I can't seem to figure out what the W means. An example is Mick Moore's USA # on his 352 being W453198 (I hope you don't mind me using your number as an example Mick, it is a beautiful truck and the first one that I say that stood out to me in the picture section).