Can you give us a more specific link to what you're asking about? All of Texas had been a Spanish colony a long time ago, which means the Spanish king at the time gave that land to white people to settle. But that would have been before Mexico, before Texan independence and before Texas became a state. But I doubt any significant claims remained in the 1950s based on the original Spanish land grants. Is that what you're referring to?

Edit: Didn't notice you specified the king. Fernando VII died in 1833, when Texas was still part of Mexico. His land grants would have been irrelevant by the 1950s, if anybody still owned land Fernando gave them, it was because their claim satisfied the laws of Mexico, independent Texas and then a century of American territorial, state and federal law prior to the 1950s. So it seems like a weird question, I doubt anybody in the 1950s was relying primarily on Fernando's land grants in court. Am I missing something?

Just to clarify, is the question asking who was given 500 acres of land by the king of Spain, or is it asking about the factuality of the claim that the USDA helped coerce and force out Spanish farmers (who were given their land by the king of Spain)?

I don't know the answer to either, but your question is a little ambiguous to me.