Hello everybody. May I explain some things?
I live in Spain, and yours is the typical "passed as Spanish, but made-in-China crap" syndrome.
Joker (or JKR) used to be a well regarded Spanish manufacturer, but moved production to China not long ago. JKR (your navaja is most likely from this line of theirs) is the sub-par brand of Joker (Like Magnum is the sub-par brand from Böker) and is pure crap. Bad F&F, crappy HT and steel, horrible tolerances and overall quality...nothing comparable to the few products that are still made here in Spain.
The other brand, Esposito, is oriented towards tourists, museums, recreation, etc. They don't make any real edged tool or weapon, they replicate dull maces, axes, knives, swords, etc. and are typicaly found in touristic places, towns, and so.
It's not there are not any great navajas from great manufacturers here, but unluckyly you all have chosen the worst of the worst, cheap Chinese reproductions from classic Spanish designs.
I own many navajas, both original and modern produced, and they are excellent knives.
From the classic Pallares (the Spanish equivalent of the Opinel) to the old-timers from the 1800s and early 1900s and the factory produced Aitor, Nieto and Muela brands and products.
You just have to search, search well, and avoid the foreign stuff. Hard to do if unfamiliar with the Spanish brands and their history. Sorry to hear that, guys.
It truly is a unique design, cool and versatile at the same time!