This modification was based on the heavy barrel mod for the Mateba 2006M, as evident by the added weights on the two sides of the barrel frame.
In reality, this mod is almost impossible to acquire, being made and imported in quantities even smaller than the base weapon itself. By the time Mateba filed for closure, only about less than a hundred units were ever marketed and became exceedingly hard to find.
Being a heavy barrel, it is particularly weird for this modification to improve Accuracy instead of Stability.
"Pesante" means "heavy and ponderous", which can be used to describe the largest and heaviest barrel option of the Matever.