A couple of years ago I found a featherweight at an estate sale. Clean as could be, few scratches and included was the receipt for the last cleaning.
It and the case smelled terrible. At that time I was told the odor comes from the animal glue used to cover the case and having the case closed was no help.
We took the machine out of the case and let it sit in an airy place. Tried a lot of different things but time seemed to be what it took. Sponged the case down. Leaving it open carried it in my hot car or left in my hot garage for the summer. Tried the soaps suggested but that smell was almost worse. Later used foot Odor-eaters (charcoal) in the case and still do. It has been closed about 6 months so I just checked to see how it was doing. Good. Be patient and leave the lid open for good long while. At that time it was also suggested to use black shoe polish on the case cover. I did not have to do that but it might be worth a try on a really damaged case. Good luck and enjoy your featherweight.