I think Wb is Wood Block. See last page here:
http://www.aosa.org/documents/Guidel...hestration.pdf. Sure, it's all in capital here, but what would the "W" in "Wb" be, otherwise?
EDIT: I see him playing contrabass in the booklet, so I guess Contrabass was right after all. It's weird, because I can't find any reference that says "Wb" is an abbreviation of it. If you can find a reference, please link to it here!
I agree with Vn being Violin.