Answer

These are made of polypropylene.
 The stopper sizes (e.g. 14.5/23) refer to the following: First number = widest part of stopper in mm (14.5mm in this example) NOT including the cap; Second number = the length of the stopper in mm (23mm in this example) NOT including the cap - i.e. if viewed from the side it is the length of the exposed part only of the central section.
 They are best measured using  vernier callipers.