As a follow up to this, i've looked at another translation of the Walker note and that translator uses the term "post office box" in place of where it says "mailbox" in the WC translation of the note. That translator uses the term "post office box" in both places.
Still though, I thought Oswalds PO box was in the Terminal Annex building in Dealey Plaza and not up at Ervay street which is a good bit away from Dealey Plaza.