None of my 20+ Willmers have any flaws in the stems, nor does the material seem appreciably more or less durable or resistant to oxidation than on other makes. I’m skeptical of the recycled rubber anecdote but stranger things have happened.
To my eye it appears that on the high grades their stems were hand cut. On their non-graded lines they may well be pre-molded. On my most often smoked Willmer I have no doubt that it has a pre-molded stem. I have several unsmoked Willmers, both graded and not, still in their original boxes, that may hold some clues. I’ll take a look later to see if there are any indications as to molded or cut.
Without a doubt Willmers varied quite a bit with respect to quality over time. I have several examples of one of their later models where the length of the stummel varies by up to a half inch between pipes.
Ultimately, except for those unsmoked examples I’ve found, all of my Willmers get a lot of use in the rotation and are dependable, enjoyable pipes. My most cosmetically flawed Willmer is perversely enough one of the best smokers in my entire collection.
I, of course, reserve the right to be completely full of it.
Ren