how can i fix an old pump bb gun

Question
My son got an old bb gun from an old man he said it was from the 1930's if I remember right. It is in good shape but the pump does not work. The old man had in the past taken it apart but when he was done putting it back together there was an extra piece. The piece looks like it is maybe a rubber band but it is  black and it is shaped like a circle it is hard and won't bend The man had it taped to the barrel so it wouldn't be lost.  Where does this piece go in the gun?    
Please help thank you very much.  
                   Sincerely  Roberta

Answer
Hello Roberta. You have a very good candidate for a wall hanging. The BB guns from the 1930's, if in good working condition, can sell to a collector for a few dollars (from very few $$ to $$$$) depending on Maker and Model. It will be a miracle if you were to find parts to fix it. However, there are web sites that handle a few parts for the earlier Name Brand guns. Google search for 'air rifle parts' and you might find the part you need. Do a little research and I am sure you will come to a conclusion of what to do with it. They do make nice window box display's. Thank you for asking for me. Charles Gage