Quite interesting this mystery was just not clarified earlier. Dipping lever to any plastic maybe sounds strange, but this special "coat" goes all around rounded leverŽ end nicely, so it is closed at the end. So hard to say this could be made by plastic tube.
On the other hand, dipping would make any casting mark ( but there are no signs of it) and this process needs propably more time in production than to heat any stretching closed one side tube.......
Even dipping, or even tube, IŽ m wondering what makes this coat so hard and keeping its shape even the moisture gets inside of it, so just not sticked perfect on the lever.