Okey, in the light of this thread being featured on the main page just in time for the easter holidays I took a more serious approach and delayed my return to the family for one more hour of research. After some deep digging I'v learned, that the German manufacturer of hot water bottles "fashy" takes pride in having "revolutionised" the manufacturing process to the use of "thermoplastic" materials instead of rubber in 1968.
In fact the process today will be a form of blowmoulding, the best hints can be found in this french patent filing from 1978:
Today one can find some pictures of "hot water bottle mould" on Alibaba as expected:
This as far as I could get without actualy visiting a plant. Sorry mates, still no clue on the Sean Penn time travel,
which is a much more involving matter, altogether.
Happy Easter (Holidays), everyone !
I am not young enough to know everything.