Balancer replacement 1/2 " 27" 2640 Milgard cica 1992

It depends on the depth of the bottom nylon shoe. This can be thought of the heel-to-toe measurement, just below and perpendicular to where the metal channel ends. The Series 351 has a depth of 1", whereas the Series 390 has a shoe that is 1-1/8" deep. Measure the depth on your old balance's shoe to determine which you need.