Hi Michael,
You'll encounter these often in estate collections here in the Southwest. I've seen them described in multiple curatorial collections as "Spanish Colonial" and have represented them as such myself. In fact, I have a pair currently on loan to a local museum.
It is my understanding these were used from the 17th-18th C. onwards in New Spain territories. I have one pair with a mark that was attributed to a Mexico City maker and dated ca. 1750. As I understand it, it was not uncommon for Spanish cannon to be melted down to be recast as utilitarian implements, these stirrups included.
While often described as "Conquistador stirrups," I personally think it is more likely that a majority of these were used by the merchant-class and colonial settlers. Either way, one cannot dispute the functionality provided by a protective brass/bronze shell.
I also believe (as has been stated) these were produced well into the 20th century. The example you show seems to indicate honest use and wear.
Regards,
Chris