It appears to be a rarely-used word for a metal alloy containing molybdenum.
Most of the references I found to the term were not public access and did not describe what "polybdenum" meant in the abstract. However, I also found a Chinese patent notice, the English translation of which includes this (emphasis mine):
"The carbon steel, aluminum and molybdenum diffused heat exchanger is characterized in that the inner and the outer walls of the carbon steel heat exchanging tube are compounded with carbon steel, aluminum and molybdenum diffused alloy layers; base material surface layers at the parts of the tube plates, the end covers, etc. "
Since the title of the patent includes the term "polybdenum" but does not mention it again in the abstract, it is reasonable to conclude that the word refers to these alloy layers.
Interestingly a number of the papers that come up appear to be Chinese in origin, which suggests that's where the term is usually used. A Google Ngrams search returns no results, so it's clearly an extremely rare term (Google Books does not contain an ecopy of The Dharma Bums, so that's why it doesn't appear as a result).
As suggested by user Gareth Rees, many grades of stainless steel contain molybdenum, including a lot of cookware. So it seems reasonable to infer Ray's bottle is made of a stainless steel containing molybdenum.