That plate they mount onto with the holes is normally part of the upright support, spotwelded on or similar. No obvious signs of it having been removed on yours, just not there. Not sure what's going on there.
Here's a pic of mine, should be the same centre support for 77-81 models. The plate is centred of the bolt holes that the horn brackets secure to and apart from a little extra strength and rigidity the only thing it does is line the horn tabs up in the plate holes so they don't swing down oever time and bumps. Should be easy to add a plate with holes and bolt up at same time of just locate horns without it.