I might be out to lunch on this one, but I think it's because most people handle banknotes with their right hand, and hold it from the centre of the note or to the right of the centre...
I'm left-handed when writing, but I still give and receive money with my right hand. If I am given a note horizontally, I grasp it from the centre or to the right of centre. If I am given a note vertically, I always grasp the right-hand side. I think this is because I hold my wallet in my left hand, and holding a banknote the way I do makes it easier to put the note into my wallet rightside up instead of upside down. I notice that most cashiers and bank tellers give you notes face up and with the portraits facing you.
Then again, if my theory is right, then the notes from ANY series, not just the 1935, should be more soiled on the right than the left...it could be that for the 1935 series, all of the portraits are on the left hand side, and people back then were rabid Royalists and didn't want to get thumb-prints all over the British Royal Family?
