Howdy Pards,
I'm sure no history expert so read this fer what it is, hearsay,opinion, and in general a regurgitation of things I've read other places.
The Scalloped part your taking about is commonly refer to as a "Spanish notch" so it would seem to make sense that many knives of Spanish heritage and/or inspiration may have a notch included at the will of either the maker or customer.
I have read folks claiming that the notch is a "tactical" feature included for the purpose of catching the blade of an opponent which would protect your own hand and provide an oppertunity to twist the opponents blade out of his hand.
My take is that it's purely a decorative thing included to add some interest and class to what is basically a big ole plain kitchen knife. Embelishment of tools and weapons has been around as long as the tools themselves to add value, distinction, and personalization to what may otherwise be a non-descript blade.
I also wouldn't dismiss the chance that once upon a time there was a knife fight where this type of thing actually did happen with a Spanish notch saving the day by catching the opponents blade. It certainly would have become a legendary in short order.
Again this info is just my opinion and relies on NO quotable facts that could be verified.
Good discussion Pards ! Josh