The base turn button was usually screwed into the wooden base of older models of Singer sewing machines such as the 201k etc. It’s job was to keep the head in place when you are sewing. It can be ‘turned’ to allow you to tip the head back on its hinges so you can gain access to the underside of your machine to clean, oil and adjust etc.
We have a limited supply of these old buttons – some are complete and some are part components.
If you’re in need of one it’s probably a good idae to get in touch first to make sure we have exactly what your’re looking for.