I'm a total novice in terms of bicycles and I've been given an old bike from a friend. Half way through cycling home I had to dismount because the back wheel slipped out of alignment and jammed against the chainstays on the left hand side. The back wheel has the normal QR lever on the left hand side, but is held on on the right by a tiny silver nut just over half a centimeter in diameter, sitting behind a little washer. Can anyone confirm that this is, as it seems to me to be, a cowboy "mend" and is it likely that this is the cause of the wheel drifting out of true? Is there any other reason this might happen and can I buy a proper fastener or a new pin anywhere?