for use in a blow molding machine for containers , especially plastic bottles , a production line is provided for thermally pretreating preforms transported by holders , where each holder is loaded with at least one exchangeably fixed fitting such as a shielding plate . to reduce the exchange period for the holders the fitting is secured by a quick - change element which remains on the holder during the exchange and is adjustable on the holder between a secured position and a release position by the application of an external force , and which can be adjusted manually and / or mechanically .