A tradition from Nantucket that you may find in other New England homes; the mortgage button. When the mortgage was paid off, the papers were burned and the ashes placed in a prominent stairway's newel post, then the post topped with a piece of scrimshaw, often with the owner's initials etc but originally was plain. Now often made of ivory, not whale bone, and is common to be installed upon sale or construction and not stuck down until mortgage paid off. Photo and more info from NinaHelman. Forget scrimshaw, even ivory buttons run around $150... but just found the below on the 'bay... owner says it is plastic, could actually be ivory. For $7.50 might be worth finding out... (Item is handturned on Nantucket natch'.)
Below, an example of a squared newel post, sans button, via OldHouseJournal.