The mounting brackets for the horns have a dimple and the hole for the attaching bolt. These line up with the two holes for each horn in the core support. The horns when thoroughly cleaned or glass beaded will have two "HI" and two "LO" on each respective horn. The HI can only be properly mounted on the driver's side of the radiator and the LO on the passenger.
Before you take the time to clean them up, test them to make sure they are working. If you use any kind of media to blast them, tightly stuff a paper towel into the horn to prevent the media from getting in. You can then use a pliers to pull the towel out before painting.