Answer:
According to the the first results I found when I searched for it, they both represent the ironic betrayals of the salesman. The "stocking" represents the salesman's betrayal towards his emotionally neglected wife (using it to fuel his own egoistic affair); he is losing his happiness to a vague hope. The “rubber tube” ironically represents the fact that the car, water heater, and other conveniences he works so hard for are actually killing him.
Step-by-step explanation:
I have not read the book, I just researched the two symbols. Combine my answer with your own knowledge of the story, as it may not be perfect.