Never mind guys, I was able to release the stuck clamps.
It took some extreme measures like taking the whole clamps apart, and then exposing the metal to some heat. After a few turns, the clamps were free. Some polishing, cleaning and after applying a really thin coat of lubricant, everything is working as expected!
Tried stringing a few rackets, and the clamps as well as the clamp bases are working wonderfully! An amazing deal for 400 USD (might seem quite a high price, but I live in Argentina, were everything costs twice as much as USA).