These stirrups are known as "Conquistador" stirrups, implying that they were taken to South America when it was first colonised by the Spanish in the 16th Century. This is probably not true - as far as I can find out none are depicted in paintings of that time, and none have been found in any of the contemporary shipwrecks off the Florida coast. Also they are generally not found in Spain itself, suggesting that they were developed in South America after the colonisation.
I've also had a short Portuguese lesson from a kind Brazilian gentleman who had seen the website
"although the general meaning of "conquistador" in Portuguese is "conqueror", it is often used to mean "charming" (as a gent willing to conquer ladies' hearts). Unless these stirrups were meant specifically for military use I would bet the last one to be the correct meaning. ... some Eberle products (late 19th, early 20th c.) used to have quite romantic names."
Another contribution (thank you) said :
At flea market years ago, I was told that there are two versions of these stirrups. The Conquistador model stirrups were made from melted church bells and have a nice "ring" to them when thumped. The other , slightly plainer model was made from melted down cannons by the troops of the Emperor Maximilian, and are therefore a couple of hundred years "younger"...
All information / ideas gratefully received