Easy answer...Your car has a 1973-74 header panel. They switched the directions of the slots for some reason. Your only solution would be to use nuts/bolts or a screw and a clip nut.
You should also have a reinforcement plate on the verticle upright for mounting the horns (73-74) that's the quickest way to tell them apart. 
Thanks for the quick response! I'll do some checking....
