Double action are easy to spot - they have a screw on cap on both ends of the shock. I watched for years on ebay before I got a pair. I would look for the caps in the ebay picture, then would check the part number. Be sure to look up the part number out of the Master Parts Catalog. It lists the Chevrolet number plus the Delco number. The double acting shocks look similar for the 1/2 ton vs larger trucks but they have different part numbers.
