... ring or “X” ring will greatly reduce the wearing and thus prolongthe service life by 3-5 times. ... 2.Model:420V,428V, 428VM,520V, 525V, 530V, 530VM, 630V, 630VM,428V-X,428VM-X ... ,520V-X,525V-X,530V-X,530VM-X,630V-X ... is made in USA.6.O-ring and X-ring are made in Tianwan.7. ...