Hi Fernando,
Ethnographic or not, I've always considered the plug bayonet a very intriguing edged weapon, and particularly those from Spain, which were likely the longest enduring in use. While as military weapons they of course became obsolete in the 18th century, as hunting arms they continued. Also, it was characteristic practice in Spain to use salvaged sword blades, much in the manner of dirks, to create these traditional weapons.
This being the case, it would appear that the mounts on this piece are relatively modern, and the blade does seem to have some age, probably early 19th century. The notched carving on the back of the blade would suggest that characteristic seen on many Andulusian examples, as well as the decorated brass ricasso plate also a favored feature (see "The Plug Bayonet", R.D.C. Evans, West Yorkshire, England, 2002, p.174).
Although these plug bayonets obviously are not able to be used in the intended manner in the barrels of more modern guns, they have gained status as a hunting weapon reminiscent of the traditional form, worn in hunting knife fashion.
Interesting piece Fernando, and an edged weapon form seldom discussed so glad to see it posted!
All the best,
Jim