The main problem with glass parts is the mold they are made from is only good for a limited number of curing cycles. The heat and shrinkage during the cure process cause them to distort. As with any tooling the cost to replace the molds is high so the makers have a tendency to run too many parts before replacing the mold. This results is the mis-fitting parts described above. Assuming the Masters the molds are made from are dimensionally correct, properly braced & stored, a part from a new mold will be far superior to the 100th part out of the same mold.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.