I'm not sure you have a 73 harness, I think all 71 & up use the 5-blade. I doubt if yours came from the factory that way.
If you have a 5-blade relay use the '73 diagram to check that it is good, the relay portion works the same as a 3-wire, (I'm not sure if their terminals are in the same position) the extra spades are for a key alarm so they can be ignored..
Check that you have power coming into the relay's 12V terminal of the fuse block, check that you have continuity to wire #24 (your reference) from the relay output terminal of the fuse block, and check that when the horn button is depressed that a ground is created to the relay's ground terminal of the fuse block. When all of those conditions are met your horns should work.
I have not had to deal with this personally but have read that any 3-wire 12V automotive relay can be wired into the fuse block using jumpers to make a working system.