The main problem regarding the bolts is that in Spain they are considered " fundamental parts ", therefore, when the gun is destroyed, ( destruction of guns in Spain is another issue we could talk for several days ), they also destroy all " fundamental parts ". All " non fundamental " parts, such as stocks, firing pins, springs, trigger mechanisms, sights ... do not need to be destroyed by law, and can be stripped off the gun before the destruction takes place. Sometimes they do, sometimes they dont. Unfortunately most often they don't and the gun is thrown complete to the melting pot of a steel manufacturing company or to the crusher.
So finding a bolt is quite difficult, but sometimes you might find a compassive police officer that might be willing to sneak one, but is never a sure thing. Obviously this makes getting them in big quantities totally impossible.
For the same reason if somebody sells a Destroyer, he or she has to sell it complete. There is no selling only the gun without the bolt, unless you can prove to the police that you have somehow lost the bolt and filled a police report about it.
So, how those 50 guns ended up in the US
without the bolt ?
Some military warehouses keep the rifles separated from their bolts. Due to the administrative chaos that Spain is, the police or the army are no exception, and with the passing of the years those bolts were probably moved, covered by a dusty tarp, or only God knows what. One good day, someone found the carbines without the bolts and decided to sell them for real cheap to an exporter, without even bothering to look for the bolts. The rest of the history is easy to imagine.
What will happen to those 50 bolts ?
One day they will find them, and from that point they could do anything, most likely they will destroy them, but depending on who finds them they could even end up being sold in the black market.